Device and method for outputting a private image using a public display

ABSTRACT

Disclosed are a device and a method for displaying a private image on a public display device. Image sequence pattern is generated for the private image and the corresponding masking image. The masking image is made from the dynamic inverse image of the private image, based on the refresh rate of the display device and the image sequence pattern. The masking image can screen the private image more effectively. The private image and the masking image is displayed on the display device according to the image sequence pattern.

TECHNICAL FIELD

The present invention relates generally to an apparatus and method foroutputting a private image and, more particularly, to an apparatus andmethod for outputting a private image to a public display so as toprevent unauthorized persons from viewing the private image.

BACKGROUND ART

Portable terminals, such as mobile phones, Personal Digital Assistants(PDAs) and notebook computers, and desktop Personal Computers (PCs) arefrequently utilized in public places. At this time, contents on displaymonitors can be viewed by all the persons located within visibledistances from the display monitors. Due to such a security problem,when a computer is used for the writing of text, mail, chatting or videowatching and a user does not want other persons to view the contentsthereof, the use of a computer is limited. A privacy problem may arisenot only when computers are privately used but also when confidentialdocuments are written using computers in corporations or governmentoffices. Besides, the security problem exists in various fields. Forexample, Automatic Teller Machines (hereinafter referred to as “ATMs”)are deployed in public places, so that security information, such as thekey inputs of passwords of ATM users and transaction details on screens,can be easily exposed. Accordingly, it is useful to develop a displaythat provides private information to an authorized user on a publicmonitor and prevents unauthorized persons from viewing the privateinformation on the same public monitor.

Early Liquid Crystal Display (hereinafter referred to as “LCD”) monitorshave narrow viewing angles, so that screens look dark when viewed fromlocations offset from locations in front of the screens. Since theabove-described feature is inconvenient in terms of general use,technological efforts have been made to widen the viewing angle thereof.However, the above-described feature is somewhat advantageous in termsof security, and a LCD monitor having the above-described feature may beregarded as the earliest private display. A display having microblocking members developed from such a concept is disclosed in U.S. Pat.No. 5,528,319 (invented by Russel). However, this scheme is defective inthat contents on a display are completely exposed to persons locatedbehind a user. Sceptre Co. in 1998. This product using polarization isconstructed so that a polarizing plate is eliminated from the interiorof a typical LCD monitor and then a user views the security monitorusing polarizing glasses. However, this product is inconvenient forgeneral use because a user can only view the contents when he wears thepolarizing glasses, even in circumstances in which privacy is notimportant. Furthermore, an unauthorized person wearing glasses havingsimple polarizing characteristics, such as typical sunglasses, can viewthe contents, so that security is low. As a result, this product failedin the market, and the manufacture of this product was stopped. MMI Co.(http://www.man-machine.com/invisivw.htm) somewhat improved thistechnology and commercially sold a product, to and from which apolarizing plate may be selectively attached and detached, in 2001.However, this product is still defective in that an unauthorized personwearing simple polarizing glasses, such as sunglasses, can viewcontents.

The most perfect private display is a Head-Mounted Display (hereinafterreferred to as an “HMD”). However, the HMD is expensive, heavy to wearand power-consuming because both a display and an optical system areaccommodated in glasses.

For displays that are not private displays but technologically relateddisplays, multi-screen displays, in which a single display shows twodifferent types of images, and users having shutter glasses view theirown images, respectively, are disclosed in Korean Pat. Appl. No.1991-0000391 (filed by Samsung Electronic Co. and entitled“multiple-screen display device and viewing device for monitors”),Korean Pat. Appl. No. 1997-04686 (filed by Samsung Electronic Co. andentitled “video apparatus for simultaneous viewing of two screens”) andKorean Pat. Appl. No. 1999-0051191 (filed by Hoyseung Choi and entitled“apparatus for simultaneous reproduction of multi-type images”). Thetechnology, in which the multi-screen display is applied to a game, isdisclosed in U.S. Pat. No. 5,963,371 (assigned to Intel Corporation).The multi-screen displays cannot be considered to be private displaysbecause persons other than persons wearing the shutter glasses can viewsome of the contents displayed on the displays. That is, the shutterglasses are used only to block other images, and there is no provisionfor a means for protecting private images.

In the present specification, a single display screen distinguished bythe vertical sync of a monitor is referred to as a monitor frame, and asection of image data is referred to as an image data frame. The size ofa single image data frame may be identical with or different from thatof a single monitor frame. A private image (hereinafter referred to as a“P image”) is the private, non-public image of an authorized user. Amasking image (hereinafter referred to as an “M image”) is an image thatblocks the P image of an authorized user.

A private display for protecting private information using shutterglasses is currently being implemented. Since this private display isinexpensive, is light to wear and can be developed further, this schemeis regarded as the most competitive method currently. The privatedisplay should fulfill all three performance conditions, including ‘uservisual perception performance,’ ‘naked eye security performance’ and‘anti-peeper security performance.’ The ‘user visual perceptionperformance’ is the performance that allows an authorized user toclearly view an image without visual inconvenience or strain, the ‘nakedeye security performance’ is the performance that prevents unauthorizedpersons having no shutter from clearly viewing an image, and the‘anti-peeper security performance’ is the performance that preventsunauthorized persons or intentional peepers with a shutter from clearlyviewing an image.

In the present specification, a shutter opening/closing sequence statevalue (state information) is the value indicating the extent ofopening/closing, and a shutter opening/closing sequence is the sequenceof state values corresponding to an image sequence and represented like[0, 1, 0, 1, 0, . . . ]. A shutter opening/closing signal is the signalthat is transmitted to control the opening/closing operations of ashutter in accordance with the shutter opening/closing sequence, andgenerally includes one or more of shutter opening/closing sequence statevalues.

A P/M image sequence scheme (hereinafter referred to as a “SunMicrosystems' scheme”) disclosed in U.S. Pat. No. 5,629,984 by SunMicrosystems, Inc. is illustrated in FIGS. 1 a and 1 b. This scheme isone of synchronous schemes, which opens/closes shutter glasses whilealternately displaying a private image data frame P and a masking imagedata frame M in accordance with vertical sync Vsync that is the framesync of a monitor, thus allowing only a user possessing the shutterglasses to view private images. The basic alternation of the P and Mimage frames is that the P and M image frames are displayed one afteranother, as shown in FIG. 1 a. Alternately, as shown in FIG. 1 b,alternation having a ratio of 1:m (m=1, 2, . . . ), in which, wheneverthe P image frame is displayed one time, the M image frame is displayedm times, has been proposed. The sequence of U.S. Pat. No. 5,629,984, inwhich P and M images are alternated with each other at a ratio of 1:1,has weak ‘anti-peeper security performance’ because the sequence can beeasily interpreted when a peeper learns a refresh rate and conforms tovertical sync. Furthermore, since only a white flash image is generatedas a masking image, it is difficult to conceal a private image.Meanwhile, the sequence, in which P and M images are alternated witheach other at a ratio of 1:m (when m is larger than 2), fulfills ‘nakedeye security performance.’ However, as m becomes larger, the number ofprivate images becomes smaller and the difference between an openingduration and a closing duration becomes greater, so that ‘user visualperception performance’ is degraded. Furthermore, since the alternationsequence of 1:m is a periodic sequence, a peeper can learn m by learninga refresh rate, conforming to a vertical sync and performing scanning,the sequence can be learned without difficulty, so that the alternationsequence of 1:m has weak ‘anti-peeper security performance.’Additionally, in the patent, a shutter opening/closing signal istransmitted without being encrypted at every time of opening/closing, sothat it is easy for a peeper to intercept the shutter opening/closingsignal.

A P/M image sequence scheme (hereinafter referred to as a “IBM'sscheme”) disclosed in GB Unexamined Pat. Publication No. 2360414 A isillustrated in FIG. 1 c. This scheme is one of asynchronous schemes,which transmits a private image data frame P and a masking image dataframe M to a monitor in accordance with Data sync Dsync. The data syncDsync is not in accordance with monitor frame sync Vsync, image dataframes are asynchronously displayed, and data sync Dsync is synchronizedwith shutter glasses. In this scheme, the display periods Dcycle of theP and M image frames are varied and thus encrypted, so that ‘anti-peepersecurity performance’ is improved. The shutter opening/closing signal isencrypted by binding a plurality of shutter opening/closing sequencesfor a certain period and a plurality of image frame display periods, andthe encrypted shutter opening/closing signal is transmitted to ashutter. The shutter decodes the encrypted shutter opening/closingsignal, and then synchronizes the decrypted shutter opening/closingsequences and image frame display periods with the data sync of imageframes using the timer of the shutter. Accordingly, ‘anti-peepersecurity performance’ is increased for a peeper to intercept the shutteropening/closing signal. The density of light intensity varies with theregions of the monitor due to the asynchronization between an imageframe and a monitor frame, so that it is inconvenient for a user to viewimages. The non-uniformity of an image occurs in a boundary region wherethe P image frame is alternated with the M image frame due to thecombination of an asynchronization with the finite response time of theshutter, so it is inconvenient for a user to view images. Furthermore,the probability that a P image frame is displayed on a specific region(upper end portion) of a monitor is increased, so that ‘naked eyesecurity performance’ and ‘anti-peeper security performance’ aredecreased. Even in an asynchronous scheme, an image frame repeatedlyalternates between a P image frame and an M image frame, so that theprobability that a peeper views a private image through tuning becomeshigher. The manufacturing costs of the shutter and power consumption areincreased in that the timer is required at a receiving side for thetransmission of the encrypted shutter opening/closing signal andsynchronization. The masking image data of an M image frame is generatedas a simple random pattern image, so that the present scheme is noteffective in concealing a private image.

The private display of Mitsubishi Electric Research Laboratory (MERL)disclosed in February of 2002 at a website(http://www.merl.com/papers/TR2002-11/) is based on a synchronousscheme. The private display of MERL generates the inverse image ofprivate image data as masking image data using the time integration ofthe eye and displays the inverse image, so that unauthorized person ismade to view a uniform gray image, which is the mean image of theprivate image and the inverse image, thereby improving ‘naked eyesecurity performance.’ In order to improve ‘anti-peeper securityperformance,’ a P/M image frame sequence is randomly generated andprovided. In particular, using a high-price fast shutter such asFerroelectric Liquid Crystal (FLC), shuttering based on frames ratherthan pixels has been proposed. In the MERL's scheme, when a reverseimage is generated, the gamma of a monitor is taken into consideration.However, the understanding of human visual perception is insufficient,so that an incomplete reverse image is generated. Furthermore, a longtime is required to calculate a reverse image per frame, so that areal-time system cannot be implemented with the MERL's scheme.

In the MERL's scheme, when a reverse image is generated, the gamma of amonitor is taken into consideration. Furthermore, since a long period isrequired to calculate a reverse image per frame, a real-time systemcannot be implemented with the MERL's scheme, so that the MERL's schemeis applied only to a private display for still images. Furthermore, inthe private display of MERL, a disturbing image having a cognitivemeaning is used as a masking image. In this case, by reducing thedynamic range of a private image compared to a masking image, adisturbing image is made to be more clearly viewed. In this case, thedynamic range is the concept identical with the difference between themaximum and minimum values of a color space, the range of monitorbrightness, and the range of voltage applied to a monitor. However, inthe MERL's scheme, a particular photo image, which is effective inconcealing a particular still private image, is used as a disturbingimage, so that the MERL's scheme cannot be a systematic and strategicdisturbing image generating method and does not take human visualperception characteristics into consideration. Furthermore, the MERL'sscheme does not present a method of providing a disturbing image perframe in real time, so that the MERL's scheme can be applied only to aprivate display for still images.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the prior art, and an object of the presentinvention is to generate a masking image that is more effective inconcealing a private image in view of human visual characteristics.

Another object of the present invention is to generate a masking imagethat conceals a private image in real time.

A further object of the present invention is to provide a method ofcombining a private image and a masking image capable of improving bothuser visual perception performance and security performance in a privateimage output apparatus.

Yet another object of the present invention is to provide a privateimage output method that displays two or more different image frames fora single monitor frame period while fulfilling user visual perceptionperformance, thus improving ‘anti-peeper security performance.’

In order to accomplish the above object, the present invention providesan apparatus for outputting a private image using a public display,comprising means for generating at least one private image; means forgenerating at least one masking image that masks the private image;means for generating an image sequence of the private image and themasking image; and means for outputting the private image and themasking image to the display according to the image sequence; whereinthe masking image generating means generates a dynamic inverse image ofthe private image as the masking image according to a refresh rate ofthe display and the image sequence.

Additionally, the present invention provides an apparatus for outputtinga private image using a public display, comprising means for generatingat least one private image; means for generating at least one maskingimage that masks the private image; means for generating an imagesequence of the private image and the masking image; and means foroutputting the private image and the masking image to the displayaccording to the image sequence; wherein the masking image generates adisturbing image based on human visual perception characteristics as themasking image.

Additionally, the present invention provides an apparatus for outputtinga private image using a public display, comprising means for receivingat least one private image from an external server or generating aprivate image by itself; means for receiving at least one masking imagefrom an external server; means for generating an image sequence of theprivate image and the masking image; and means for outputting theprivate image and the masking image to the display according to theimage sequence.

Additionally, the present invention provides an apparatus for outputtinga private image using a public display, comprising means for receivingat least one private image from an outside of the apparatus; means forgenerating at least one masking image that masks the private image;means for generating an image sequence of the private image and themasking image; and means for outputting the private image and themasking image to the display according to the image sequence.

Additionally, the present invention provides an apparatus for outputtinga private image using a public display, comprising means for receivingat least one private image from an outside of the apparatus; means forgenerating at least one masking image that masks the private image;means for receiving an image sequence of the private image and themasking image; and means for outputting the private image and themasking image to the display according to the image sequence.

Additionally, the present invention provides an apparatus for outputtinga private image using a public display, comprising means for generatingat least one private image; means for generating at least one maskingimage that masks the private image; means for generating an imagesequence of the private image and the masking image; and means foroutputting the private image and the masking image to the displayaccording to the image sequence; wherein the masking image generatingmeans comprises a color table storage unit having a color table forgeneration of the masking image and an image conversion unit generatingthe masking image with reference to the color table for generation ofthe masking image.

Additionally, the present invention provides a method of outputting aprivate image using a public display and a shutter, comprising selectinga rule for mixing private at least one image and at least masking imageto fulfill security performance and user visual perception performance;generating an image sequence of the private image and the masking imageaccording the mixing rule; generating a shutter opening/closing signalcorresponding to the image sequence to fulfill the user visualperception; outputting the private image and the masking image to thedisplay according to the image sequence; and opening/closing the shutteraccording to the shutter opening/closing signal.

Additionally, the present invention provides a method of outputting aprivate image using a public display and a shutter, comprisinggenerating an image sequence of at least one private image and at leastone masking image using a method of inserting phase change aperiodicityby limiting a maximum allowable repetitive number of unit repetitiveperiods each composed of at least one private image and at least onemasking image; generating an image sequence of at least one privateimage and at least one masking image to fulfill user visual perceptionperformance; outputting the private image and the masking image to thedisplay according to the image sequence; and opening/closing the shutteraccording to the shutter opening/closing signal.

Additionally, the present invention provides a method of outputting aprivate image using a public display and a shutter, comprisinggenerating an image sequence of at least one private image and at leastone masking image using a method of limiting a maximum allowablerepetitive number of images having a same characteristic; generating ashutter opening/closing signal corresponding to the image sequence tofulfill user visual perception performance; outputting the private imageand the masking image to the display according to the image sequence;and opening/closing the shutter according to the shutter opening/closingsignal.

Additionally, the present invention provides a method of outputting aprivate image using a public display and a shutter, comprisinggenerating an image sequence of at least one private image and at leastone masking image using a method of inserting phase change aperiodicityby limiting a maximum allowable repetitive number of unit repetitiveperiods each composed of at least one private image and at least maskingimage while limiting a maximum allowable consecutive number of imageshaving a same characteristic; generating a shutter opening/closingsignal corresponding to the image sequence to fulfill user visualperception performance; outputting the private image and the maskingimage to the display according to the image sequence; andopening/closing the shutter according to the shutter opening/closingsignal.

Additionally, the present invention provides a method of outputting aprivate image using a public display and a shutter, comprisinggenerating an image sequence of at least one private image and at leastone masking image using a method of inserting phase change aperiodicityby limiting a maximum allowable repetitive number of unit repetitiveperiods each composed of at least one private image and at least maskingimage; generating a shutter opening/closing signal corresponding to theimage sequence to fulfill user visual perception performance; outputtingthe private image and the masking image to the display according to theimage sequence; and opening/closing the shutter according to the shutteropening/closing signal; wherein the generation of the shutteropening/closing signal is performed in such a way that the shutter isopened/closed at an intermediate state in a vicinity of a location wherethe phase change aperiodicity is inserted.

Additionally, the present invention provides a method of outputting aprivate image using a public display and a monitor, comprising selectinga rule of arrangement of at least one private image and at least onemasking image to mask a specific region of the monitor; generating animage sequence of the private image and the masking image according thearrangement rule; generating a shutter opening/closing signalcorresponding to the image sequence to mask a specific region of themonitor; outputting the private image and the masking image to themonitor according to the image sequence; and opening/closing the shutteraccording to the shutter opening/closing signal.

Additionally, the present invention provides a method of outputting aprivate image using a public display and a monitor, comprising selectinga rule for arranging private images and masking images to be alternatedwith each other at least two times for a single monitor frame;generating an image sequence of private images and masking imagesaccording the arrangement rule; generating a shutter opening/closingsignal to correspond to the image sequence; outputting the private imageand the masking image to the monitor according to the image sequence;and opening/closing the shutter according to the shutter opening/closingsignal.

Additionally, the present invention provides a method of outputting aprivate image using a public display and a monitor, comprising selectinga rule for arranging private images and masking images to be alternatedwith each other at least two times for a single monitor frame;generating an image sequence of private images and masking imagesaccording the arrangement rule; generating a shutter opening/closingsignal to correspond to the image sequence; outputting the private imageand the masking image to the monitor according to the image sequence;and opening/closing the shutter according to the shutter opening/closingsignal; wherein masking images having different characteristics areoutput to regions of the monitor.

Additionally, the present invention provides a method of outputting aprivate image using a public display and a monitor, comprising selectinga rule for arranging private images and masking images to be alternatedwith each other at least two times for a single monitor frame;generating an image sequence of private images and masking imagesaccording the arrangement rule; generating a shutter opening/closingsignal to correspond to the image sequence; outputting the private imageand the masking image to the monitor according to the image sequence;and opening/closing the shutter according to the shutter opening/closingsignal; wherein a private image or connecting image is output for aresponse time of the shutter.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter, with reference to the attached drawings, the embodiments ofthe present invention are described in detail. For ease of description,in the drawings, the same reference numerals are used to designate thesame or similar components.

FIGS. 1 a to 1 c are drawings illustrating the conventional sequences ofprivate images and masking images;

FIG. 2 is a system configuration diagram of a notebook computerembodiment to which the present invention is applied;

FIG. 3 is a system configuration diagram showing an embodiment of asoftware method-dedicated driver in accordance with the presentinvention;

FIG. 4 is a block diagram showing an example of a video controller shownin FIG. 3;

FIG. 5 is a system configuration diagram of software type applicationlevel embodiment in accordance with the present invention;

FIGS. 6 a to 6 c are configuration diagrams showing examples ofwired/wireless communication interfaces located between computer bodiesand shutter opening/closing means;

FIG. 7 is a system configuration diagram showing an embodiment of ahardware type external controller in accordance with the presentinvention;

FIG. 8 is a block diagram showing an example of the external controllerof FIG. 7;

FIG. 9 is a system configuration diagram showing an embodiment of afully independent hardware type external controller in accordance withthe present invention;

FIG. 10 is a system configuration diagram showing an embodiment of acombined SW/HW type external controller in accordance with the presentinvention;

FIG. 11 is a block diagram of a circuit for generating a masking imageusing a non-linear inverter in accordance with the present invention;

FIG. 12 a illustrates the spatial contrast sensitivity function of humanvisual perception for luminance and color, and FIG. 12 b illustrates thetemporal contrast sensitivity function of human visual perception forluminance and color;

FIG. 13 is a view illustrating a time delay between the center andsurround mechanisms of a human visual nerve;

FIG. 14 is a diagram illustrating a general process of the generationand perception of an image;

FIG. 15 is a view illustrating a typical monitor transfer function;

FIG. 16 is a view illustrating a typical human visual perceptionfunction;

FIG. 17 is a view illustrating a typical monitor pixel response;

FIG. 18 is a view showing an example of a pattern used to generate adynamic inverse image through a dynamic pattern test method according tothe present invention;

FIG. 19 is a view showing the status of a general color table;

FIG. 20 is a diagram showing a color table changing method according tothe present invention;

FIG. 21 is a flowchart illustrating a disturbing image generating andmanaging method according to the present invention;

FIGS. 22 a to 22 c are diagrams showing the spatial and temporalfrequency characteristics of a human visual perception neuron, humanvisual perception and typical image data, respectively;

FIG. 23 is a diagram showing a CIE Lab color space;

FIG. 24 is a diagram showing a human visual perception process;

FIGS. 25 a to 25 c are diagrams showing a process of generating adisturbing image through the mixing of image components in accordancewith the present invention;

FIG. 26 is a flowchart illustrating a process of generating a sequenceof private images and masking images and a shutter opening/closingsequence in accordance with the present invention;

FIGS. 27 and 28 are diagrams showing an aperiodicity insertion sequencemethod according to the present invention;

FIGS. 29 to 34 are diagrams illustrating a process of generating asequence of private images and masking images and a shutteropening/closing sequence in accordance with the present invention;

FIGS. 35 and 36 are diagrams illustrating the relation between therelative light transmittances of a shutter and corresponding shutteropening/closing sequence state values;

FIGS. 37 a and 37 b are diagrams illustrating the response time of ageneral monitor pixel;

FIGS. 38 to 40 are diagrams illustrating a shutter opening/closingsequence compensating for the slow response time of the monitor pixel inaccordance with the present invention;

FIG. 41 is a diagram illustrating partial screen private display inaccordance with the present invention;

FIG. 42 is a diagram illustrating a region division image arrangementsequence method in accordance with the present invention;

FIG. 43 is a diagram illustrating the private image grouping method of aregion division image arrangement sequence;

FIG. 44 is a diagram illustrating the variation of private imagegrouping in accordance with the present invention;

FIG. 45 is a diagram illustrating the maximum repetitive period sequencemethod of the region division image arrangement sequence;

FIG. 46 is a diagram illustrating a sequence of private images andmasking images, a shutter opening/closing sequence and a shutteropening/closing sequence light response;

FIG. 47 is a diagram illustrating the processing of the boundary regionof a private image and a masking image on the basis of a shutteropening/closing sequence light response in accordance with the presentinvention;

FIG. 48 is a diagram illustrating an example of the processing of theboundary region of a private image and a masking image to compensate forthe difference in the density of light intensity in accordance with thepresent invention; and

FIG. 49 is a diagram illustrating another example of the processing ofthe boundary region of a private image and a masking image to compensatefor the difference in the density of light intensity in accordance withthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[System Configuration]

A private display can be widely used for a variety of monitor displaydevices, such as a desktop PC, a notebook computer, a PDA, a mobilephone, a Television (TV), a Digital Versatile Disk (DVD), an ATM/CashDispenser (CD), and a door lock information input device. The presentinvention is described below on the basis of an embodiment of a PCmonitor that is a typical one of the above-described devices. Thoseskilled in the art can easily modify the embodiment and apply themodification to other display devices. Generally, other display deviceshave structures simpler than that of the PC monitor, so that a privatedisplay device can be implemented with some of the indispensableelements of the PC monitor embodiment excluded.

FIG. 2 is a basic configuration diagram of an embodiment applied to anotebook computer. As shown in this drawing, the embodiment includes acomputer 104 equipped with a monitor 102, a shutter opening/closingmeans 106 performing optical filtering, a wired/wireless communicationmeans 108 connecting the computer 104 with the shutter opening/closingmeans 106, and private display software (not shown). The computer 104may include the private display software by storing the private displaysoftware in computer-readable memory. The computer 104 allows onlyauthorized persons to view a private image in such a way as to displaythe private image and a masking image for masking the private image onthe monitor 102 in response to a user's request or spontaneously, and totransmit a shutter opening/closing signal to the shutter opening/closingmeans 106, and thus operate the shutter opening/closing means 106. Inthis case, the computer 104 collectively refers to a variety ofinformation devices that display images on monitors, such as a desktopmonitor, a notebook computer, an PDA, a mobile phone, a TV, a DVD, anATM/CD, and a door lock information input device.

The shutter opening/shutting means 106 may be a mechanical means, or aphotoelectric means, such as a liquid crystal shutter. The shutteropening/closing means 106 may be fabricated in various forms, such asglasses having one or more shutter lenses, a shutter structure having asupport, or a shutter cap. FIG. 2 illustrates a shutter opening/closingmeans in a shutter glasses form.

In the case where the computer 104 is used in bright circumstances, aproblem may arise in that ambient light reflected from the monitor 104is incident on the user's eye and the contrast of a monitor image islowered. In the present embodiment, an ambient light blocking filter(not shown), such as “3M™ Privacy Computer Filter” sold by 3M Company,is attached on the front of the monitor 102. The ambient light blockingfilter is an optical filter, the transmittance of which depends on anincident angle and which blocks light having an incident angle greaterthan a certain angle. The ambient light blocking filter consists of afilm having microlouvers functioning like a window shade, and blockslight incident at an angle greater than a certain angle. In anotherembodiment, the brightness of a monitor is improved by attaching abrightness enhancement film, such as “Vikuiti™ Display Enhancement Film”sold by 3M Company, to the front of the monitor. When such a filter isattached to the front of the monitor, ambient light incident on andreflected from the monitor 102 can be effectively blocked, but ambientlight directly incident on the shutter opening/closing means 106 cannotbe blocked. To block ambient light directly incident on the shutteropening/closing means 106, the ambient light blocking filter is attachedto the shutter opening/closing means 106. When the ambient lightblocking filter is attached, the contrast of an image that the userperceives is increased, but the entire brightness of the monitor issomewhat decreased. It is designed that the ambient light is attached ina bright place and detached in a dark place.

FIG. 3 is a system configuration diagram showing an embodiment of asoftware method-dedicated driver according to the present invention. Thesoftware method imports that all functions, except the functions of ashutter unit 306 and a transmission/reception interface 308, areimplemented using software. A dedicated driver 310 refers to a driverthat accesses a video controller 312, such as a graphic card, andimplements private display in real time, independent of a graphic driver314 in a PC 302.

A private display control block 318 is composed of a securityperformance control means, an encryption means, a user authenticationmeans and a management means. The private display control block 318authenticates a user accessed through a user interface 320, and sets andmanages a security level depending on a user's authentication level anda user's input. A method of authenticating the user may be implementedin such a way that the user is authenticated based on the inputidentification number and password of the user. In another embodiment,the user authentication may be implemented in such a way as to connectan authorized shutter opening/closing means without an identificationnumber (hereinafter referred to as an “ID”) and a password. In stillanother embodiment, the user authentication may be implemented in such away as to connect an authorized shutter opening/closing means andreceive an authorized ID and a password. The authentication of anauthorized shutter opening/closing means and a genuine product isperformed using the serial numbers of products stored in the Read OnlyMemory (ROM) of the shutter unit 306. The private display control block318 receives monitor information on the basis of a user's authenticationlevel and a display's security level, and controls an image data framesequence generating means 322, a shutter sequence and shutteropening/closing signal generating unit 324, and a masking imagegenerating means 326. A monitor information acquiring means 328 readsinformation about the resolution of a monitor, refresh cycle time,vertical sync and horizontal sync.

The image data frame sequence generating means 322, the shutter sequenceand shutter opening/closing signal generating unit 324 and the maskingimage generating means 326 generate an image data sequence, a shutteropening/closing sequence and a shutter opening/closing signal, and amasking image according to the user's authentication level, thedisplay's security level and a user's additional selection,respectively. The shutter sequence and shutter opening/closing signalgenerating unit 324 generates a shutter opening/closing sequence insynchronization with the image data frame sequence, and generates ashutter opening/closing signal at current time according to the shutteropening/closing sequence.

The dedicated driver 310 provides a masking image generated in themasking image generating means 326 to video memory 328 according to thegenerated image data frame sequence, spontaneously generates a maskingimage according to the instruction of the masking image generating means326, and controls the change of a color table in real time. Thededicated driver 310 controls image transmission to the monitor 304 bymaking the video controller 312 switch the private image memory blockand the masking image memory block.

The transmission/reception unit 308 transmits a shutter opening/closingsignal to the shutter opening/closing means 306. Thetransmission/reception unit 308 can transmit an encrypted shutteropening/closing signal to an authorized user using the encryption means(not shown). The transmission/reception units 308 and 336 can beimplemented through a wired link, such as a Universal Serial Bus (USB)and a serial link, or a wireless link, such as an InfraRed (IR) link anda Radio Frequency (RF: Frequency Modulation (FM), Amplitude Modulation(AM) or Bluetooth) link. The video controller 312, such as a graphiccard, is provided with video memory, and displays an original videoimage, which is received from the graphic driver 314, and a maskingimage, which is received from the dedicated driver 310, on the monitor304 according the image data frame sequence.

As illustrated in the drawing, the shutter opening/closing means 306includes a transmission/reception unit 310, a decoder/authenticationmeans 330, a shutter controller 332 and a shutter 334. Thetransmission/reception unit 310 receives the encrypted shutteropening/closing signal transmitted from the transmission/reception unit308, and transmits the encrypted shutter opening/closing signal to thedecoder/authentication means 330. The decoder/authentication means 330acquires a shutter opening/closing sequence state value by decryptingthe encrypted shutter opening/closing signal, and the shutter controller332 fully opens/closes or half opens/closes the shutter 334 according tothe shutter opening/closing sequence state value.

The display security level is set to ‘naked eye security performance’for an unauthorized person, and to ‘anti-peeper security performance’for an unauthorized person having another shutter. In general, as thedisplay security performance becomes higher, ‘user visual perceptionperformance,’ such as the visual comfort and clarity of the user,becomes lower. Such display security level can be defined in variousways. In an embodiment, at a first level, an unauthorized person cannotperceive even the approximate type of user private images even thoughthe unauthorized person views a monitor for a period longer than acertain period. This level is the strictest security level, in which,for example, it cannot be found out whether the user uses a wordprocessor or views moving images. At a second level, the approximatetype of user images can be perceived when an authorized person views amonitor for a period longer than a certain period. However, theunauthorized person cannot perceive a part of the content of userprivate images. For example, the unauthorized person can perceive thatthe user are viewing moving images, but cannot find out whether the useris viewing a movie or engaging in chatting. At a fourth level, anunauthorized person can accurately perceive a part of the content ofuser private images when the unauthorized person views a monitor for aperiod longer than a certain period. However, the authorized personcannot learn most of the contents of the user private images. Forexample, the unauthorized person can somewhat perceive the content of adocument word-processed by the user, or the content of a motion picture.At a fifth step, an unauthorized person can perceive a considerable partof the contents of the user private images. However, the unauthorizedperson feels uncomfortable in viewing the user private images. Inanother embodiment, an additional performance index indicating theextent, to which user private images and a masking image are perceivedby unauthorized person, may be added to the performance level. In thiscase, more various display security levels can be set.

FIG. 4 is a view showing the video controller 312 and the video memory328 using functional blocks. As illustrated in the drawing, according tothe position of the image memory, private image memory blocks areindicated with P1˜Pm, and masking image memory blocks are indicated withM1˜Mn. P1˜Pm are blocks managed by the system, and M1˜Mn are maskingframes generated for masking by the dedicated driver 310 that is akernel driver. Each of the memory blocks is selected by a method, suchas flipping, and transmitted to the monitor 304 through a color table402 functioning as an image value (RGB value) converter and RAMDAC(RFG)/DVI monitor connection unit 404. In an embodiment, each of thememory blocks P1˜Pm and M1˜Mm corresponds to the size of a singlemonitor frame. The dedicated driver 310 generates a thread, and updatesimage data in the masking image memory blocks M1˜Mn randomly set in thevideo controller 312 whenever a spare period occurs in the CentralProcessing Unit (CPU). The dedicated driver 310 checks the vertical syncstatus register 406 of the video controller 312 in a polling method, orcauses a selected image frame to be displayed in such a way as to hook avertical sync interrupt Vsync_INT generated in the vertical syncgenerating circuit 408 and applied to the system, to select one of theimage frames P1˜Pm and M1˜Mn in synchronization with the vertical syncsignal, and to record the starting address of the selected frame in theframe starting address register of the video controller 312.

In another embodiment, an image frame selected in synchronization with acertain horizontal sync signal may be made to be displayed. In a colortable changing method to be described below, there can be obtained theeffect of converting a private image into a masking image in such a wayas to perform random image value conversion by randomly changing thecolor table using the dedicated driver while a private image block istransmitted to the monitor. In still another embodiment, a filter drivercan be inserted instead of the dedicated driver of FIG. 3. The filterdriver functions like the dedicated driver 310, and is involved andoperated like a filter when the graphic driver 314 accesses the videomemory 328. In still another embodiment, an alternative driver methodmay be employed. This method integrates the function of a dedicateddriver with the function of an existing graphic driver and performscontrol using a single driver.

Masking images may be classified according to masking characteristics.The masking characteristics are represented using a superscript notationas follows. An original image-derived image is represented using M^(i),an intentionally disturbing image is represented using M^(d), and aconnecting image frame for smooth shutter opening/closing or a smoothimage change is represented using M^(b). For a masking image in which anoriginal image-derived image and a disturbing image are mixed with eachother, the type in which the original image-derived image is main andthe disturbing image is added to the original image-derived image isrepresented using M^(i), the type in which the disturbing image isprincipal and the original image-derived image is added to thedisturbing image is represented using M^(d), the type in which and theoriginal image-derived image and the disturbing image have the sameimportance or the importance of the two images is not clear isrepresented using either M^(i) or M^(d).

The original image-derived image is a masking image derived from aprivate image that is an original image, which includes an inverseimage, the differential or integral image of the original or inverseimage, the filtered image of the original or inverse image, and theshifted image of the original image. The disturbing image collectivelyrefers to masking images except for an original image-derived image. Theconnecting image frame for smooth shutter opening/closing or a smoothimage frame change is an image frame that is inserted so as to enablesmooth shutter opening/closing or a smooth image frame change. Theconnecting image frame refers to a blank image frame, a uniform grayimage frame or a uniform color image frame.

In another embodiment, the principal functions of the dedicated drivermay be implemented at an application level, as shown in FIG. 5. In thiscase, the dedicated driver 508 is in charge of transmitting a shutteropening/closing sequence through the transmission/reception unit 510 andcontrolling the real-time change of the color table. An image controlmeans 514 at an application level provides a masking image to videomemory 518 according to the generated image data frame sequence, andspontaneously generates the masking image according to the instructionof the masking image generating means 524 and provides the masking imageto the video memory 518. The image control means 514 at the applicationlevel makes the video controller 516 control the transmission of animage to the monitor 504 by switching a private image to a masking imageand vice verse according to the generated image sequence. In anembodiment, the image control means at the application level may beimplemented by a flipping method using DirectX. In another embodiment,the image control means 514 produces a virtual desktop and displays thevirtual desktop to the user. In still another embodiment, the imagecontrol means 514 can expand a desktop and control the movement of theoriginal point of the desktop.

FIG. 6 is block diagrams showing the wired/wireless communicationinterface located between a body 502 and the shutter opening/closingmeans 306 shown in FIGS. 3 and 5. The communication interface can beimplemented with a wired link, such as a USB, IEEE 1394 or a seriallink, or a wireless link, such as IR or RF (FM, AM or Bluetooth). In anembodiment, in the USB wired link shown in FIG. 5, a USB system receivesa signal indicating the opening/closing of a shutter and the extent ofthe opening/closing thereof from the USB port 602 of a PC. The USBsystem obtains voltages required for the control of the shutter in sucha way as to receive a power of +5V from the USB port 602 of the PC andboosts the power of +5V to powers of +12V and +15V using a power module606. The shutter controller 608 receives a control signal from a USB CPU610, and applies a voltage to the shutter 612 in the glasses. Thecontrol signal can be implemented using modulation, such as a PulseWidth Modulation (PWM), or with a Direct Current (DC) voltage. Theembodiment of the wired link, in which a decoder/authentication means632 is additionally included in a shutter unit 626 is constructed, asshown in FIG. 6 b. In the embodiment of FIG. 6 a, the shutter controller608 is included in the transmission unit 604. In contrast, in theembodiment of FIG. 6 b in which a private image can be viewed using anauthorized shutter, a shutter controller 634, along with adecoder/authentication means 632, is included in a reception unit 626and an encrypted shutter opening/closing signal is transmitted to thereception unit 626. In another embodiment of the wireless link shown inFIG. 6 c, a transmission unit 646 including a transmitter 652 isconnected to the USB system of a computer, and a reception unit 648includes a shutter 660, a receiver/decoder/authentication means 654, apower module 656 and a shutter controller 658.

FIG. 7 is a system configuration diagram showing an embodiment of ahardware type external controller in accordance with the presentinvention. In comparison with a software type controller in which allthe functions, except the functions of a shutter and a transmission andreception interface, are implemented with software, a hardware(hereinafter referred to as “HW”) type controller refers to thecontroller in which core functions, such as sequence control, thegeneration of a masking image and the combination of a private imagewith a masking image, are implemented with hardware. For the hardwaretype controller, there are various embodiments, including an internaltype controller, an independent card type controller, a graphic cardincorporating type controller, a graphic chip incorporating typecontroller and a monitor incorporating type controller. Theseembodiments have common features in which core functions, includingsequence control, the generation of a masking image and the combinationof a private image with a masking image, are implemented with circuits.These embodiments can be changed to each other by a simple change incircuits according to the location of a core function circuit relativeto the monitor 304 and the computer 702. The external type controller isthe type of controller in which the core function circuit is implementedoutside the monitor 304 and the computer 702, the independent typecontroller is the type of controller in which the core function circuitis implemented in the form of a card to be inserted into the computer,and the graphic card incorporating type controller is the type ofcontroller in which the core function circuit is added to a graphicchip. The graphic chip incorporating type controller is the type ofcontroller in which the core function circuit is added to a graphicchip, and the monitor incorporating type controller is the type ofcontroller in which the core functions are added to a drive circuit in amonitor. Of these various type controllers, the HW and external typeembodiment shown in FIG. 7 is described below as a representative.

The HW type controller connects a video controller 710 to a monitor 304through a core function circuit 704 rather than directly connecting thevideo controller 710 to the monitor 304. In the SW type controller, adedicated driver performs the functions of sequence control, maskingimage generation and transmission, and shutter opening/closing meansconnection, while in the HW type controller, the dedicated driver 714simply performs the functions of software and hardware connections. Inthe HW type controller, sequence control, the generation of a maskingimage, and the combination of a private image with a masking image isimplemented with a single external circuit 704. That is, the corefunction circuit 704 includes a sequence controller 732 performingcontrol according to the image sequence of private images and a maskingimage and a shutter opening/closing signal, a transmission/receptionunit 738 transmitting the shutter opening/closing signal to a shutteropening/closing means 306, a masking image generating means 734generating the masking image, and a monitor interface 736 transmittingthe private images transmitted from the video controller 736 accordingto the image sequence and the masking image transmitted from the imagegenerating means 734 to the monitor 304.

FIG. 8 is a block diagram of an HW type external controller. Theexternal controller 802, in which a core function circuit isimplemented, is provided with control information (drive or stopinformation) or sequence information from the COM, PS2 or USB port ofthe computer and notifies the computer of the state of the externalcontroller 802. Furthermore, the external controller 802 receives a RGBsignal, a DV1 signal, a vertical sync signal Vsync and a horizontal syncsignal Hsync from the VGA output terminal 804 of the computer. Thevertical sync signal Vsync or horizontal sync signal Hsync causes aninterrupt to the microprocessor 806 of the external controller 802, andthe microprocessor 806 set a signal to be output by applying a sequenceprovided by the computer or generated therein. At this time, theexternal controller 802 determines opening/closing or the extent ofopening/closing by applying an appropriate DC voltage or a PWM signalhaving a duty corresponding to the DC voltage using a shutter controller808.

A masking image signal generator 812 manipulates the RGB or DV1 signalof a private image obtained from the VGA output terminal 804 andgenerates a random disturbing signal by itself. In the HW typecontroller, the generation of the masking image is implemented with thegeneration of an inverse image using an inverting circuit with respectto the RGB or DV1 signal of a private image, the generation of anoriginal image-derived image using a differentiator circuit, anintegrator circuit or one of various filters, the generation of a randomnoise masking image, the generation of a disturbing image using a selfimage storage module, or the generation of an image using theappropriate combination thereof. The masking image signal generator 812functions as a converter for converting a RGB value. A method ofgenerating an inverse image is implemented in such a way as to invert asignal on the basis of a random reference value between the maximum andminimum values of a RGB signal, and thus generate an analog signal. Thereference value is randomly regulated in view of human visualcharacteristics, and thus can have a bias from an average value. A blanksignal source 814 can generate a gray frame by applying a constantvoltage to the monitor 304 irrespective of the RGB signal, and can beused for the purpose of a smooth screen change. Furthermore, there isused a method of making a masking image brighter than a private image byallowing the masking image to be transmitted to the monitor 304 througha boosting circuit. The HW type controller can implement boosting andthe generation of random noise without the burden of calculating time.

FIG. 9 is a system configuration diagram showing an embodiment of afully independent HW type external embodiment according to the presentinvention. The fully independent HW type controller is the type ofcontroller in which most of the functions of private display areimplemented with independent hardware. In this type controller, thevideo controller 710 of a computer system 902 is not directly connectedto a monitor 304, but the video controller 710 is connected to themonitor 304 through independent hardware 904. Since most or all of thefunctions of a private display are implemented with the independenthardware 904, there is an advantage in that a need to change a softwarepart for implementing the private display to correspond to the computersystem 902 is reduced.

FIG. 10 is a system configuration diagram showing an embodiment of acombined SW/HW type external controller. This system combines the SWtype with the HW type, and thus implements a private display. Inparticular, the principal point of the system is that a dedicated driver1004 generates a masking image using a masking image generating means524 in a hardware fashion, generates a masking image using a maskingimage generating means 734 in a software fashion, and integrallyprovides the masking images. In an embodiment, a basic masking image isgenerated in a software fashion, and random noise, a differential orintegral effect or a filtering effect is applied to the basic maskingimage in a hardware fashion.

A method of generating an inverse image using a non-linear inverter andadding random noise to the inverse image is illustrated in FIG. 11. InFIG. 11, reference numeral 1102 designates a non-linear inverter,reference numeral 1104 designates a random signal generator, andreference numeral 1106 designates a signal combiner. As shown in thisdrawing, when a masking image is generated, required time can beconsiderably reduced compared to the case where the masking image isgenerated using software, so that the masking image can be generated inreal time.

[Human Visual Characteristics Related to Private Display]

In the private display in which the frame of a private image and theframe of a masking image are continuously changed, the temporal andspatial frequency characteristics, temporal integration and temporalinhibition out of human visual characteristics should be takenseriously. Of the human visual characteristics, the temporal and spatialcharacteristics are presented by a Spatial Contrast Sensitivity Function(hereinafter referred to as “SCSF”) shown in FIG. 12 b and a TemporalContrast Sensitivity Function (hereinafter referred to as “TCSF”) shownin FIG. 12 b. FIG. 12 a illustrates the SCSF for luminance (∘) and color(□), and FIG. 12 b illustrates the TCSF for luminance (∘) and color (●).As shown in the drawings, in a low spatial frequency and low temporalfrequency region, visual perception is sensitive to both luminance andcolor, particularly to color. In a low spatial frequency andintermediate temporal frequency region, an intermediate spatialfrequency and low temporal frequency region, and an intermediate spatialfrequency and intermediate temporal frequency region, visual perceptionis sensitive to both luminance and color. In a high spatial frequencyand high temporal frequency region, visual perception is sensitive toluminance, but insensitive to color. A Critical Flicker Frequency(hereinafter referred to as “CFF”) is the highest temporal frequencypossible to respond, and is about 55 Hz.

In the meantime, according to eye physiology, between retinas and brainsexist visual nerve signal pathways, including a magnocellular pathway(hereinafter referred to as “M pathway”), a parvocellular pathway(hereinafter referred to as “P pathway”) and a K pathway. Thecharacteristics of the M and P pathways that correspond most of thevisual nerve signal pathways are represented in Table 1. Temporalresolution is best in a cone cell located immediately around a fovea andconnected to the M pathway.

The temporal resolution of a cone cell connected to the P pathway of thefovea is intermediate, and the temporal resolution of a rod farthestfrom the fovea is worst. The rod has the best temporal sensitivity.TABLE 1 M pathway (M-cell) P pathway (P-cell) Color discrimination noneexist Neuron response temporal continuous Axon diameter large smallTemporal resolution high frequency low frequency Spatial resolution lowhigh Retinal input region around fovea fovea Specialization movementdetailed pattern, color

A human eye performs a visual perception using time-integrationsampling. General mechanical signal sampling picks up signals atsampling points, whereas time-integration sampling samples valuesobtained by integrating signals within time-integral intervals. Thetime-integral intervals adaptively vary with stimulus intensity,stimulus size, background intensity, phase coherence and contrast.Basically, time integration is primarily performed at a photoreceptor.Visual nerve pathways extending from the photoreceptor to brains havedifferent integral intervals, and information integration is finallyperformed in the brains. That is, time integrations are performed foradaptively variable time-integral intervals on the basis of a collectionof such multiple time integrations.

The receptive field of a visual nerve cell is composed of receptors thatreceive stimulus inputs. The center-surround receptive field of aganglion cell or LGN cell receives an excitatory input from a centerreceptive field and an inhibitory input from a surround receptive field.The center-surround receptive fields of the cells have center-surroundantagonism with respect to luminance, but do not have it with respect tocolor (refer to FIG. 13 b). Table 2 shows the structure of the P pathwayof a P cell. In this case, a somewhat time delay exists between a centerresponse and a surround response (refer to FIG. 13 a). At a low temporalfrequency, a time delay is shorter than the period of a response signal,so that it generates synergism with respect to luminance and antagonismwith respect to color (refer to FIG. 13 c). If the period of a responsesignal is continuously decreased by the increase of a temporalfrequency, 180 degree phase shift occurs between the center response andthe surround response, so that synergism is generated with respect toluminance and antagonism is generated with respect to color (refer toFIG. 13 d). Due to such physiological time suppression structure, thevisual nerve is insensitive to the luminance variation of a low temporalfrequency, sensitive to the luminance variation of 5˜20 Hz, insensitiveto the color variation of a low temporal frequency and insensitive tothe color variation of 20 or more Hz. TABLE 2 Color opponency AntagonismRed and green cell L − M cone cell none (center + surround) Blue andyell cell S − (L + M) cone cell none (center + surround) Luminance (B-W)cell L + M cone cell exist (center + surround)[Generation of Inverse Image]

An inverse image can be generated using time integration as a maskingimage for concealing a private image. If a private image and an inverseimage are provided within a time integral interval, a human integratesimages for the time integral interval, so that the mean image of theprivate image and the inverse image is perceived.

The simplest inverse image that can be thought of is a static image. Ina luminance-color domain, a pixel image value of a static inverse valueis a value that is obtained by subtracting a pixel image value of anoriginal image (here, the original image indicates a private image) fromthe maximum pixel image value. For example, in a RGB domain, one pixelimage value of the original image is (R′, G′, B′) and the maximum pixelimage value is (R′m, G′m, B′m), the pixel image value of the staticinverse image is (R′i, G′i, B′i)=(R′m-R′, G′m-G′, B′m-B′). However, inthe static situation in which original image and inverse image framesare continuously alternated with each other, as in the private display,a static inverse image does not form a genuine inverse image due to thecomplicated visual perception characteristics of the eye, so that adynamic inverse image should be pursued.

The dynamic inverse image is calculated based on a perceptual domain.The general structure of the generation and visual perception of animage is as illustrated in FIG. 14. An image is generated in a computerthrough an image editing program or the input of a user, or the objectof a light energy domain is photographed by a camera, and thus has thevalue Ga of a computer domain. In general, the value Ga is the valuegamma-corrected with respect to the intensity and luminance of the lightenergy domain. The values of the luminance and color of the light energydomain are expressed by tristimulus values, as in a CIE XYZ or RGBdomain. The image value of the computer domain corresponding to the RGBdomain is gamma-corrected Ga=(R′, G′, B′). The image value Ga of thecomputer domain is output while being converted into a monitor inputvoltage, and is displayed through a monitor. The process is expressed asbeing converted into the value I of the light energy domain. Thetransfer function of this process is called a monitor transfer functionf_(rm), and relation I=f_(rm)(G_(a)) is established. The monitortransfer function f_(rm) assumes the form of FIG. 15. In general, themonitor transfer function f_(rm) is relatively correctly modeled with apower function such as I=G_(a) ^(rm). The intensity and luminance of thelight energy domain are perceived by the visual perception functionf_(a) between the luminance of a human and perceived brightness, andrelation V=f_(a)(I) is established. The visual perception function f_(a)assumes the form of FIG. 16. In general, the visual perception transferfunction f_(a) is relatively correctly modeled with a power functionsuch as V=I^(a). When the monitor transfer function and the visualperception function are modeled with the power functions, multipliersγ_(m) and α are in approximately reciprocal relation. Accordingly, G_(a)and V are in approximately linear relation.

However, the relation is a result obtained in a static situation. In adynamic situation in which a pixel image value varies with time, G_(a)and V deviate from linear relation. Since the sensitivity of humanvisual perception varies with the temporal and spatial frequencies of apixel image value, G_(a) and V deviate from linear relation. When aninverse image is given as a masking image in a private display, a pixelimage value has the temporal frequency of a specific region and ischanged to that of high contrast, so that the non-linearity of G_(a) andV increases in importance. In this case, the temporal frequency isconsiderably influenced by a refresh rate and the frame sequence ofprivate images and masking images. As a result, to obtain an inverseimage in a private display, a dynamic inverse image should be obtainedwith the temporal frequency of an image taken into consideration,together with time integration.

As known from the luminance TCSF curve (FIG. 12 b) of visual perception,a sensitive drop-off at a low frequency and a sensitive rise at a highfrequency are largely attributable to a visual perception informationprocessing mechanism related to time suppression. According to the curveand related experimental results, a flash of 5˜30 Hz is perceived as thebrightest (Bruke-Barey effect). When the luminance variation frequencyof a flash exceeds CFF (about 55 Hz), time suppression has no influence,so that a mean image based on time integration is perceived.

An optimum dynamic inverse image varies with the refresh rate. When therefresh rate is higher than 2 CFF (about 110 Hz) and private and inverseimages are alternated with each other at a ratio of 1:1, the private andinverse images exist at a ratio of 1:1 in an adaptive variable timeintegral interval, so that a mean image (sum image) is perceived by thenaked eye. When an inverse image is provided, the bright portion of aprivate image become dark and vice verse, so that a frequency around arefresh rate and a refresh rate/2 becomes the principal frequency of animage. Accordingly, when the refresh rate is 120 Hz, P/M imagealternation of approximately 60 Hz occurs, so that the frequency of P/Mimage alternation is higher than CFF, that is, the refresh rate/2 ishigher than CFF, and thus time suppression has no influence.Accordingly, a mean image based on time integration is perceived. As aresult, when the refresh rate is higher than 2CFF (about 110 Hz), thereis no great difference between a dynamic inverse image calculated basedon a perceptual domain and a dynamic inverse image calculated based on alight energy domain. It is assumed that, when a time integral intervalis Δt_(s), a single pixel displays a private image at an intensity ofI_(p)(t) for Δt_(p), and a masking image, which is an inverse image, atan intensity of I_(i)(t) for Δt_(i). In this case, Δt_(s)=Δt_(p)+Δt_(i),and Δt_(p)=Δt_(i). In general, the intensity response of a pixel of aCathode Ray Tube (CRT) or LCD monitor is as illustrated in FIG. 17,I_(p)(t) and I_(i)(t) are variables. For convenience, an ideal monitorbeing energy-equivalent to that of an actual monitor and following thecharacteristics of time integration is considered. The ideal monitordisplays the intensity corresponding to an image value Ga in the form ofa rectangular waveform. Δ{circumflex over (t)}_(p) and Δ{circumflex over(t)}_(i) are calculated with representative intensity values set to theintensities of the ideal monitor Î_(p) and Î_(i). In this case,Î_(p)Δ{circumflex over (t)}_(p)=I_(p)(t)Δt_(p) and Î_(i)Δ{circumflexover (t)}_(i)=I_(i)(t)Δt_(i). With the above manipulation, the idealmonitor being energy-equivalent to that of the actual monitor isobtained. It is approximated that a single pixel displays a privateimage at an intensity of Î_(p) for Δ{circumflex over (t)}_(p), and amasking image, which is an inverse image, at an intensity of Î_(i) forΔ{circumflex over (t)}_(i). The approximation is correct because, inpractice, within a time integral interval, the same image is perceivedwithout regard to waveforms when the products of intensities and timesare same.

The intensity of the mean image of a private image and an inverse imageÎ_(d) can be simply obtained as expressed by Equation 1. When this isrepresented in the image value of a computer domain, it is expressed bythe following Equation 2. When it is represented in a general powerfunction, it is expressed by the following Equation 3. In this case, ddesignates a mean image, p designates a private image, and I designatesan inverse image. $\begin{matrix}{{\hat{I}}_{d} = \frac{{\hat{I}}_{p} + {\hat{I}}_{i}}{2}} & (1) \\{{\hat{f}}_{\gamma_{m}d} = \frac{{\hat{f}}_{\gamma_{m}p} + {\hat{f}}_{\gamma_{m}i}}{2}} & (2) \\{G_{ad}^{\gamma_{m}} = \frac{G_{ap}^{\gamma_{m}} + G_{ai}^{\gamma_{m}}}{2}} & (3)\end{matrix}$

First of all, a mean image is obtained. If the maximum values of acurrent computer domain are set to G_(a max) and G_(a min), the meanimage value G_(ad) is calculated from the following Equation 4.Thereafter, with respect to the private image value G_(ap), an inverseimage value is simply obtained from the following Equation 5 or 6.$\begin{matrix}{G_{ad}^{\gamma_{m}} = \frac{G_{a\quad\max}^{\gamma_{m}} + G_{a\quad\min}^{\gamma_{m}}}{2}} & (4) \\{G_{ai}^{\gamma_{m}} = {{2G_{ad}^{\gamma_{m}}} - G_{ap}^{\gamma_{m}}}} & (5) \\{G_{ai}^{\gamma_{m}} = {G_{a\quad\max}^{\gamma_{m}} + G_{a\quad\min}^{\gamma_{m}} - G_{ap}^{\gamma_{m}}}} & (6)\end{matrix}$

On the contrary, since a static inverse image is obtained as expressedin the following Equation 1, it is not suitable for dynamic displaycircumstances. For example, if a RGB space and 24 bit color are used ina computer domain, G_(a max)=225 and G_(a min)=0. If the gamma γ_(m) ofa monitor is 2.2, a mean image value is G_(ad)≅186 and an inverse valuefor G_(ap)=170 is G_(ai)=201. In the case of a static inverse image, amean image value is G_(ad)≅128 and an inverse value for G_(ap)=170 isG_(ai)=85 In a RGB space, respective RGB components are calculated insuch a manner. $\begin{matrix}{G_{ad} = \frac{G_{ap} + G_{ai}}{2}} & (7)\end{matrix}$

When a refresh rate is lower than 2CFF (approximately 110 Hz) andprivate images and inverse images are alternated approximatelyhalf-and-half, the frequency of the alternation of P and M images islower than CFF, so that time integration is not complete and timesuppression exerts an influence. Accordingly, the calculation of adynamic inverse image is not accurate. The inverse image should becalculated with the influence of incomplete time integration and timesuppression and the uncertainty of the modeling of a monitor transferfunction and a visual transfer function taken into consideration.However, since the process of human visual perception is not accuratelyknown, there is no limitation in calculating an inverse imageanalytically.

Accordingly, under such circumstances, a dynamic image is calculatedusing the below dynamic pattern test method. If the maximum and minimumvalues of a current computer domain are set to G_(a max) and G_(a min),respectively, a hatched portion is allocated G_(a max) and a blankportion is allocated G_(a min) in the pattern of FIG. 18 a, and thepattern and the reverse image of the pattern (the hatched portion isallocated G_(a min) and the blank portion is allocated G_(a max)) aredisplay while being alternated half-and-half with the pattern being setto an original image. When the image is viewed, the high clarity ofletter T indicates that time integration is being incompletelyperformed, and time integration is complete when letter T is notperceived. With this image, the extent of time integration at a currentfresh rate is learned. Thereafter, a rough mean image value iscalculated using the following Equation 8. $\begin{matrix}{G_{ad}^{\gamma_{m}} = \frac{G_{a\quad\max}^{\gamma_{m}} + G_{a\quad\min}^{\gamma_{m}}}{2}} & (8)\end{matrix}$

Thereafter, the hatched portion of FIG. 18 a is allocated theabove-calculated mean image value G_(ad), the blank portion thereof isallocated G_(a min), and the pattern and the reverse image of thepattern (the hatched portion is allocated G_(ad) and a blank portion isallocated G_(a max)) are displayed while being alternated half-and-halfwith the pattern set to an original image. When display is performedwhile changing the mean image value G_(ad), an image value, at which theclarity of letter T is lowest, is closest to the mean image value. Thevalue is found out and selected as the mean image value. Thereafter, thestep of obtaining dynamic inverse values G_(ai) for respective originalvalues G_(ap) is performed. A rough dynamic inverse value G_(ai) iscalculated from the following Equation 9 using the mean image valueG_(ad) obtained through the above-described step.G _(al) ^(γ,)=2G _(ad) ^(γm) −G _(ap) ^(γm)  (9)

The hatched portion of FIG. 18 a is allocated a rough mean image valueG_(ad), the blank portion thereof is allocated G_(ap), and the patternand the reverse image of the pattern (the hatched portion is allocatedG_(ad) and a blank portion is allocated G_(a max)) are displayed whilebeing alternated half-and-half with the pattern set to an originalimage. When display is performed while changing the mean image valueG_(ai), an image value, at which the clarity of letter T is lowest, isclosest to the mean image G_(ai). The value is selected as the dynamicinverse value mean image value for a given value G_(ap). In this manner,dynamic inverse values are selected and stored for all G_(ap) values (inthe case of an RGB space, all R, G, B). Although in the abovedescription, the dynamic pattern test method using the pattern of FIG.18 a was taken as an example, the above-described procedure can betested using various patterns, including the pattern of FIG. 18 b.

The above-described dynamic pattern test method is particularlyeffective when the provision ratio of private image to inverse imagesdeviates from 1:1, a refresh rate is lower than 2 CFF (approximately 110Hz), the fluorescence material of a monitor is degraded or the actualtransfer function of the monitor deviates from a modeled monitortransfer function. Furthermore, if the effect of time suppressionsomewhat remains even though a refresh rate is higher than 2 CFF(approximately 110 Hz) and private images and inverse images arealternated approximately half-and-half, the actual transfer function ofa monitor may deviate from a modeled monitor transfer function, so thata further accurate dynamic inverse image can be obtained when such adynamic pattern test method.

The private display software previously stores calculation methodsaccording to refresh rates, the display ratios of private images andinverse images and the types of monitors (CRT and LCD), and dynamicinverse values calculated using the dynamic pattern test method. Usingthese, the user uses a dynamic inverse value suitable for theenvironment of use and user's selection. Meanwhile, characteristics aresomewhat different according to monitors, so that the private displaysoftware may be provided with dynamic pattern test method softwareincluded therein.

[Color Table Changing Method]

When an inverse image is generated as a masking image, an inverse imagecan be generated by calculating inverse image values corresponding topixel image values for the respective pixels of a private image andstoring the inverse image values in a video memory. This method takes alot of calculation time because software calculation must be performedfor respective pixels and the calculated pixel inverse image values mustbe stored in the memory. This method can be applied only to the privatedisplay of a still image because it is difficult to generate an inverseimage in real time. In order to generate an original image-derivedimage, such as an inverse image, in real time, the present inventionemploys an image value conversion method using an image value (RGBvalue) converter. For a preferable embodiment of the present invention,an original image-derived image is generated in real time by randomlyconverting a color table according to a conversion rule using a colortable, which is included in a video controller, such as a graphic card,as an image value converter. When transmitting the image values ofrespective pixels stored in the video memory to a monitor, a generalcomputer system converts image values with reference to a color tablewhen transmitting the image value Ga of a computer domain to themonitor, D/A converts the converted image values, and then transmits thevalues in an analog fashion as for RGB voltage or in a digital fashionas for DVI.

Originally, the color table is used for the minute correction of animage. Generally, an image value is scarcely different from itsconverted image value. For an embodiment, as shown in FIG. 19, when animage value Ga is (R, G, B), the image value Ga is output as itsconverted image value (R_out, G_out, B_out) without big change, with aspot (in FIG. 19, when R=188, R_out=187) in need of minute correctionbeing slightly changed.

In the present invention, the conversion values of a color table arechanged according to a random conversion rule. A first color table for aprivate image and a second color table for a masking image are prepared,and the actual color table of a video controller is changed to thevalues of the first or second color table when each of the images isdisplayed. In this case, depending on the conversion values of thechanged color table, different images are generated, the differencebetween the image value Ga and the converted image value may be large,and the converted image may have opposite characteristics. For anembodiment, when Ga-correspondent conversion image values correspondingto a dynamic inverse image is entered in a color table, instead of anoriginal color table, the dynamic inverse image is displayed even thoughan original image is displayed without change. FIG. 20 illustrates anembodiment in which a dynamic inverse image is implemented using thecolor table changing method, with R=187 being used as a mean imagevalue. In this case, pixel image value Ga=(R, G, B) is output as aconverted image value according to a changed conversion rule in whichR=0−>R_out=255, R=255−>R_out=0, R=186−>R_out=188, R=187−>R_out=187, andR=188−>R_out=186. The color table changing method generates an inverseimage by changing only the few values of the color table withoutcalculating inverse values for respective pixel image values, so thatthe time required for the generation of an inverse image is short.Accordingly, the color table changing method is effective in thereal-time generation of an inverse image.

In the color table changing method, when a color table is changedaccording to a random rule, dynamic inverse values cannot only beobtained, but a random converted image also can be obtained according tothe changed color table. For an embodiment, when Ga-correspondentconversion image values are set to constant values for respective Gavalues, an image irrelevant to the input image of the color table can beobtained. With this method, a uniform gray image, a uniform colored grayimage and a uniform colored image irrelevant to an input image can beobtained.

In the original image-derived image calculation method in which inverseimage values are calculated for the respective pixels of a private imageand an inverse image is generated based on the calculated inverse imagevalues, for example, an original image-derived image calculated from theprivate image of the P1 memory of FIG. 4 is stored in a M1 memory and aM1 image is output to a monitor. In contrast, in the color tablechanging method, an original image-derived masking image is displayed byconverting a color table according to an original image-derived imagecalculation rule and outputting the private image of the P1 memory tothe monitor, without the direct manipulation of a video memory. Inanother embodiment, an original image-derived image is displayed bystoring the private image of the P1 memory in the M1 memory, convertinga color table and outputting the image of the M1 memory.

The conversion values of the color table are changed in synchronizationwith the vertical sync or a specific horizontal sync of a graphic card,or in a non-synchronous fashion on a driver grogram. In an embodiment inwhich the values of a color table are changed in accordance with avertical sync, the vertical sync is learned in a driver in a polling orinterrupt fashion, and the values of the color table are replaced withnew values immediately after the vertical sync is generated. In theinterrupt fashion, the interrupt of an existing graphic driver is hookedand the values of a color table are changed whenever an interrupt isapplied. Since several-ten or less microseconds are required to changethe values of the color table, the effect of the changed color table isimmediately exhibited. In another embodiment of the present invention,the real-time conversion of image values is implemented using a HW typeinverter, or a differentiating or integrating circuit as an image valueconverter.

[Generation of Intentional Disturbing Masking Image]

A disturbing image in the present invention is defined as the genericterm of masking images except for an original image-derived image, whichhas no dependence on a private image. Heretofore, a white flash image, arandom image, a still photo image, or a screen saver image has been usedas a disturbing image. However, these images are effective only inmasking specific private images, are not systematic and strategicdisturbing images capable of being used for general private images, anddoes not take human visual perception characteristics intoconsideration.

Since the white flash masking image disclosed in U.S. Pat. No. 5,629,984has a low spatial frequency and a low temporal frequency, it isdifficult for the white flash masking image to mask a private imagehaving different temporal and spatial frequencies or a cognitive meaningperceptually grouped. To effectively perform masking using a flashmasking image, a plurality of flash masking images must be displayedwith respect to a single private image, so that the ‘user visualperception performance’ at which the user views a private image, isdegraded. The random masking image disclosed in GB Pat. No. 2360414 haswide temporal and spatial frequency bands with respect to each ofluminance and color, so that the random masking image is superior to theflash masking image in masking performance. The random masking imagedoes not differentiate temporal and spatial frequency bands, to whichthe human eye is sensitive, from temporal and spatial frequency bands,to which the human eye is not sensitive, so that it is difficult for therandom masking image to mask a general private image having many signalsof a specific frequency band. Furthermore, it is difficult to mask aprivate image having a cognitive means perceptually grouped using therandom masking image having no cognitive meaning.

In the present invention, a systematic and strategic method of producingand managing a disturbing image in view of human visual perceptioncharacteristics is presented. FIG. 21 is a process of producing andmanaging a disturbing image in accordance with an embodiment of thepresent invention. First, a disturbing image with human visualperception characteristics taken into consideration is produced andstored at step 2101. The disturbing image is produced using thecharacteristic of frequencies to which visual perception is sensitive, aluminance contrast characteristic, a color contrast characteristic and acognitive content characteristic. In an embodiment, a disturbing imagecapable of effectively masking a random occurable private image isproduced. In another embodiment, a disturbing image in which a specificcharacteristic is emphasized to effectively mask a specific privateimage is produced. The file of the disturbing image may have a sizecorresponding to that of a monitor frame, or an image block unit largeror smaller than a monitor frame. In an embodiment, a plurality ofdisturbing images are produced. In another embodiment, a plurality ofdisturbing images used for respective security levels are produced.

Thereafter, if a plurality of disturbing images have been produced, anindex based on the characteristics of the disturbing images is set up orfile names are determined at step 2102 so as to allow a search requiredfor the generation of an appropriate disturbing image to be carried out.Thereafter, a private display mode is started by the user or system atstep 2103. At this step 2103, it is determined whether a disturbingimage will be used as a masking image. If it is determined that thedisturbing image will be used, it is determined whether thecharacteristics of a currently displayed private image will be examinedat step 2104. If it is determined that the characteristics will not beexamined, the process proceeds to the step 2106. If it is determinedthat the characteristics will not be examined, the temporal frequency,contrast and cognitive content characteristics of the private image areexamined and learned at step 2105. In an embodiment of the presentinvention, a content-based examination method is employed to examine thecharacteristics of the private image. For an example, the cognitivecontent characteristics of the private image are examined based on thecharacteristics of a window or an active window currently used (whethera word processor, web browser or moving image player has been used). Foranother example, application software currently being executed isexamined, and the cognitive content characteristics of the private imageare learned based on the typical characteristics of the private image.For still another example, the user may input the expectedcharacteristic information of the private image to the system or selectan expected mode based on an application to be used. For still anotherexample, a frequency characteristic is learned in real time through thefrequency conversion of the private image and the disturbing imagesuitable for the learned frequency characteristic is produced at thelater step. To reduce the burden of computing, a part of the privateimage may be frequency-converted.

At step 2106, a disturbing image generation policy is set up. The policyfor determining the level of the frequency, brightness contrast, colorcontrast and cognitive content characteristics of a disturbing imageaccording to a set display security level if the display security levelhas been set, or a random display security level if the display securitylevel has not been set, and generating a disturbing image is set up. Thepolicy includes the frequency, luminance contrast, color contrast andcognitive content characteristics of a disturbing image to be generated,and is determined based on systematized methodology. Furthermore, thepolicy includes conditions about whether a disturbing image includingtext will be generated, and what color contrast will be used. Thesystematized methodology for determining the policy is programmed andthe programmed systematized methodology is included in the privatedisplay control block 318 or masking image generating means 326. At thisstep, a disturbing image generation policy suitable for thecharacteristics of a private image is set up if the characteristics ofthe private image have been learned at step 2105, or a disturbing imagegeneration policy effective on the average is set up if thecharacteristics of the private image have not been learned at step 2105.

Thereafter, the private display control block 318 and the masking imagegenerating means 326 search for a disturbing image fulfilling theconditions of the generation policy at step 2107. For another example,the user may search for the disturbing image through the user interface320. If the suitable disturbing image has been searched for, theselected disturbing image is generated and loaded on the memory at step2108. If disturbing image files exist by the image block unit smallerthan a monitor frame, a plurality of disturbing image blocks may beloaded to construct a single disturbing image frame.

If the initial image is loaded on the memory, the disturbing image istemporally updated at step 2109. For temporal update methods, an imagecapable of occupying an entire monitor frame is loaded on the memory andupdated, one or more disturbing image parts or image blocks constitutinga monitor frame are temporally moved and re-arranged, a new image blockfile is loaded on the memory and updates a part of an image, an image ismoved by randomly changing the start address of the memory that is aflipping address during flipping, or a real-time generation programselected by the system updates an image through calculation. Thetemporal frequency of an image generated when the image is temporallyupdated is set according to the disturbing image generation policy, andthe image is updated according to a temporal variable algorithm based onthe disturbing image generation policy. To set up the temporalfrequency, brightness contrast, color contrast and cognitive contentcharacteristics of a disturbing image used in the disturbing imagegeneration method, human visual characteristics are taken intoconsideration.

FIG. 22 is diagrams illustrating the spatial and temporal frequencycharacteristics of human visual perception and typical image data. Inthese drawings, tf1, tf2, tf3, sf2 and sf3 are frequencies thatdifferentiate a low frequency, a intermediate frequency, a highfrequency and a threshold high frequency, and are 5 Hz, 30 Hz, 70 Hz,0.3 cycles/deg, 15 cycles/deg and 40 cycles/deg, respectively. FIG. 22 aillustrates the response frequency domains of P and M cells that arerepresentative visual nerve neurons, which is the physiological evidenceof human visual characteristics. FIG. 22 b illustrates visual perceptioncharacteristics obtained by integrating SCSF of FIG. 12 a and TCSF of 12b. The high frequency component of an image exceeding temporal thresholdhigh frequency tf3 or spatial threshold high frequency sf3 cannot beperceived by human visual perception. As shown in the drawings, in a lowspatial frequency and low temporal frequency domain, visual perceptionis sensitive to both luminance and color, particularly to color. In alow spatial frequency and intermediate temporal frequency domain, anintermediate spatial frequency and low temporal frequency domain, and anintermediate spatial frequency and intermediate temporal frequencydomain, visual perception is sensitive to both luminance and color. Inan other high spatial frequency or high temporal frequency domain,visual perception is sensitive to luminance but insensitive to color.

FIG. 22 c illustrates the spatial and temporal frequency characteristicsof image data. In the present invention, images are divided into textimages and non-text images according to the cognitive contents ormeanings of images. The text images include characters and symbols, andthe non-text images include photographed images, pictures and graphicimages except for text. The photographed images are images obtained byphotographing means, such as a camera, and include photographs, movies,TV broadcast images. In the present invention, a still image and amoving image are defined based on the temporal frequency characteristicof images. The still image is defined as an image in which a text ornon-text image is temporally stopped or almost stopped. The moving imageis defined as an image in which a text or non-text image is moving.Text, which is temporally moving, as well as a computer graphic movingimages, movies and TV broadcast images are defined as moving images. Aphotographed moving image, a graphic moving image and a text movingimage may have different temporal frequency characteristics. FIG. 22 cillustrates the frequency characteristics of a text image and a movingimage in a general sense. The text image generally occupies anintermediate spatial frequency, high spatial frequency and low temporalfrequency domain. The still image, such as a picture or a photograph,occupy a low to high spatial frequency and low temporal frequencydomain. The moving image occupies the widest frequency domain.

In the present invention, a disturbing image is produced at thedisturbing image production step with the temporal frequency taken intoconsideration, and the disturbing image is generated and updated at thedisturbing image generation and update step also with the temporalfrequency taken into consideration. Preferably, a disturbing maskingimage is generated to include the temporal and spatial frequencies ofthe luminance and color of a private image to be masked. In anotherembodiment, a disturbing image is generated to have all frequencycomponents to which visual perception is sensitive. In anotherembodiment, there is produced a disturbing image into which more imagecomponents of frequencies, to which visual perception is sensitive, areinserted. Since private images have considerably various frequencycomponents according to the contents of images, a disturbing image doesnot expect private images when the disturbing image is producedseparately from the private images. Accordingly, there may easily occurcases where some images are sufficiently masked while some images arenot sufficiently masked. In an embodiment, at step 2105, the temporalfrequency of a private image is examined and learned. If thecharacteristics of the private image have been learned, a disturbingimage is preferably produced to include the principal temporal andspatial frequencies of the private image and the more image componentsof frequencies to which visual perception is sensitive. For example, ifthe private image is a typical text image, a disturbing image isproduced to include an intermediate to high spatial frequency and lowtemporal frequency domain. Compared to a private image, a disturbingimage is produced to include the image components of frequencies towhich visual perception is sensitive. Using the above-described points,the frequency condition of the disturbing image production policysuitable for the characteristics of a private image are set up. Foranother embodiment, if the temporal frequency of a private image has notbeen learned, a frequency condition, which covers the image componentsof a frequency band, to which visual perception is sensitive, as many aspossible, is set up to produce a disturbing image effective on theaverage.

The disturbing image should be clearer than the private image to bebetter seen. For this, luminance and color contrast sensitivity next tothe temporal sensitivity of visual perception plays an important role.In an embodiment, the image components of a disturbing image are made tohave luminance and color contrast sensitivity. If an image of highluminance and color contrast to which human visual perception issensitive is systematically produced, the image should be produced in asystem that is quantified so that the contrast of an image isproportional to visual perception sensitivity. In the present invention,a color space that is constructed using luminance and color axes is usedas the quantified system. As well known, a variety of color spaces, suchas RGB, CMY and Yuv, exist. However, many color spaces does not take ahuman visual perception into consideration, and is not constructed to beproportional to visual perception sensitivity. For a color spaceproportional to human visual perception, there is CIE L*a*b* (simply,CIE Lab) color space that was standardized by Commission Internationalede I'Eclairage (CIE). FIG. 23 illustrates CIE Lab color space. The colorspace is constructed by arranging the visual perceptions of a luminancecell, a red-green cell and a yellow-blue cell, which are the threeindependent visual perception cells of the human visual perceptionsystem, in perpendicular axes. In the present invention, the differencebetween two image values A and B in CIE Lab color space is defined as aluminance and color difference. As the luminance color differencebecomes the larger, the contrast of two image values becomes the higher.The vertical component of the luminance color difference is a luminancedifference, and the horizontal component is a color difference. Theluminance difference is defined as luminance contrast, and the colordifference is defined as color contrast.

The image value point C of FIG. 23 is a central point corresponding tothe central value of a color space. If a monitor does not sufficientlyrepresent the luminance and color of the color space or the range ofimage values represented on a monitor is restricted through thecompression of the image values, such as a color table changing method,the central point can be moved. In the present invention, the contrastof a disturbing image is preferably determined and generated based on avisual perception-proportional color space, such as CIE Lab color space.For an embodiment, a disturbing image is generated to have the outerextreme image value of the color space on the average compared to aprivate image. A certain condition about luminance and color contrast isset up, and a disturbing image is produced and generated based on thecondition. For an example, a condition that allows the mean luminancedifference or mean color difference between a reference value, such asthe central value of the color space, and the image value of adisturbing image component to be larger than a certain value is set up.The image fulfilling the above-described condition has the outer imagevalue of the color space on the average. Since in this case, an image oflow contrast may be generated because disturbing image values are offsetto one extreme, the condition that allows the color difference betweenthe mean image value of a disturbing image component and the referencevalue to be equal to or smaller than the predetermined value. When animage is actually produced or generated, an image is produced orgenerated while it is evaluated through calculation whether the image isan image of high contrast fulfilling the condition in an embodiment, oran image is produced or generated while it is evaluated whether theimage schematically and quantitatively fulfills the above-describedcondition. In an example of the qualitative evaluation of thefulfillment of the condition, the area ration of a pair of imagecomponents, such as object/background, which form image contrast, istaken into consideration. If the area difference between an object and abackground is relatively large, the location of a mean image value ismoved into one having a larger area, so that mean contrast is reduced.In this case, the color difference between the mean image value and thereference value exceeds the certain value, so that the above-describedcondition is not fulfilled. A disturbing image is generated whilequalitative evaluation is performed to prevent the area ratio of a pairof image components from being large.

For an embodiment of the method of the present invention that producesand generates a disturbing image based on a visualperception-proportional color space, such as CIE Lab color space, theimage value of a disturbing image component is selected using the visualperception-proportional color space and the image value is convertedinto the color space image value of a computer domain, such as a RGBspace. The conversion can be accurately performed through calculation,and schematic and qualitative conversion may be performed. That is, evenif a corresponding RGB color space image value is schematicallygenerated with reference to CIE Lab color space, an intended contrastimage can be obtained.

In the present invention, a disturbing image having high contrastbetween the object and background of the image or between the luminanceand color of the image pattern is generated. Preferably, a disturbingimage having high contrast in various luminance and color is generated.For an embodiment, when the characteristics of a private image can belearned, a disturbing image having contrast higher than that of aprivate image is generated. For an example, when a private image isgeneral text, color contrast is higher than luminance contrast, so thata disturbing image having high luminance contrast is generated.Preferably, a disturbing image provided with an image component havinghigh luminance contrast and one or more image components having highcolor contrast is generated.

FIG. 24 is a view showing a visual perception process. An image having acognitive meaning is perceived better than an image having no cognitivemeaning perceptually grouped. When a stimulus 2302 excites the retina, apre-attentive process 2404, which is a high-speed visually perceptualprocess without feedback, perceives the stimulus. At this time, theperception or cognition of a shape does not occur, and the existence ofa stimulus, a variation or movement is perceived. Thereafter, a humanintentionally or unintentionally focuses and perceives the shape andcharacteristics of a part currently focused. At this time, feedback isreceived from a high-level information recognition, such as memorizingor reasoning, and is used in the perception process, and a stimulus iscontinuously collected from a stimulus source. Thereafter, perceivedcomponents 2412 are integrally perceived by being integrated through thefocused attention 2406. The focused intention 2414 chiefly perceives animage having a cognitive meaning perceptually grouped.

In terms of relation with the high-level perception process, perceptualgrouping is an important matter. According to the law of organization ofpsychophysics, the perceptual grouping is performed based on simplicity,similarity, continuation, proximity, common movement and meaning.Furthermore, the perceptual grouping is performed based on temporal andspatial phase coherence. When a phase change occurs in any part of apattern having a signal spatial frequency, the part can be betterviewed. This is due to grouping based on the spatial phase coherence. Apattern having a single temporal frequency undergoes a phase change at acertain time and a corresponding part can be better viewed. This is dueto grouping based on the temporal phase coherence. The image has beenmodulated to a frame sequence. When a phase change (non-repetitivesequence) in the frame sequence occurs while the image is presented tothe user in a repetitive alternation sequence, an image at the time canbe better viewed, and thus minute shimmering is sensed. Since a humandoes not sense a phase change within a temporal integral interval,grouping based on temporal phase coherence is closely related to thetemporal integral interval of visual perception. A perceptually groupedimage component forms a single cognitive meaning, so that theperceptually grouped image component is easily perceived as a whole eventhough a part of the image component is concealed or disturbed. Privateimages are generally grouped well. Accordingly, a masking image releasesthe grouping of the private images, so that a public image (the sumimage of a private image and a masking image) should be made to be seenas an image having no cognitive meaning or a different meaning. Themasking image preferably performs both the function of releasing thegrouping of the private image and the function of allowing maskingimages to be clearly grouped so as to cause a public image to be seen asan image having a meaning different from that of a private image.

For an embodiment, disturbing images are constructed to have differenttemporal and spatial phase coherences to release the temporal andspatial phase coherences of the private images. Original image-derivedimages including dynamic inverse images have the characteristics ofbeing mixed with private images and hindering the grouping of privateimages.

In order to mask the text of a private image when the text image regionof the private image is larger than a certain level, the disturbingimage of the present invention is made to have a text image component inat least one region. In order to mask the non-text of a private imagewhen the non-text image region of the private image is larger than acertain level, the disturbing image of the present invention is made tohave a non-text image component in at least one region.

When the edges of an image having cognitive contents are clear, visualperception more sensitively react upon the image, so that a disturbingimage component having clear edges is generated. In the case where adisturbing image including a photographed image component is produced, ageneral photographed image component has no high image contrast and noclear edges, so that the step of artificially clarifying the edges isfurther included as an embodiment. The disturbing image is processedusing an image processing technique, such as a histogram equalizationtechnique, contrast enhancement or homomorphic filtering, and thenstored. In another embodiment, a disturbing image includes a componenthaving a three-dimensional effect. In still another embodiment, adisturbing image includes an image component that causes dizziness, inwhich a high contrast pattern is repeatedly rotated, and illusion.

In the present invention, in order to obtain a disturbing image betterviewed, it is determined which type of image components are better seenwhile two types of images are alternately displayed. The disturbingimage is formed of image components that are determined to be betterseen as a result of the determination. A disturbing image is produced bycombining images having such sensitive frequency, contrast and cognitivecontent characteristics. Preferably, a disturbing image is produced toinclude high luminance and color contrast, a frequency to which visualperception is sensitive, and contents having a cognitive meaning. Forexample, a disturbing image is produced to include image components ofhigh luminance contrast having an intermediate temporal frequency and anintermediate spatial frequency and image components of high colorcontrast having a low spatial frequency rather than other frequencycomponents. To mask a pattern having a low spatial frequency in aprivate image, high color contrast is utilized and high luminancecontrast is added. To mask typical text and a high spatial frequencyimage, high luminance contrast is chiefly utilized, and a color contrastimage is added because a color assimilation phenomenon occurs.

In an embodiment, at disturbing image generation setup step 2106,different conditions may be set for the respective regions of a monitorframe. A disturbing image, in which visual perception is more sensitiveto a specific region of a monitor than other regions, is made to begenerated. For an example, a disturbing image, in which visualperception is more sensitive to the central portion of a monitor thanthe upper and lower regions of the monitor, is generated. At this time,a single disturbing image file may be loaded and a monitor frame may beformed of the loaded disturbing image file, or two or more disturbingimage blocks may be loaded and a monitor frame may be formed of theloaded disturbing image blocks.

Since an afterimage relatively frequently occurs in the lower region ofa monitor, the disturbing image of this region is formed of an image towhich visual perception is less sensitive than that for the centralregion of the monitor. Since the response of a shutter is late in theupper region of the monitor, the user can perceive some of the maskingimage, so that a disturbing image to which visual perception is lesssensitive is generated for this region in an embodiment.

In another embodiment of the present invention, a disturbing image isproduced and generated by mixing image components having two or moredifferent cognitive content, temporal frequency and contrastcharacteristics. For an embodiment in which the mixing is spatiallyperformed, spatial distribution arrangement and spatial overlappingarrangement are utilized. In the spatial distribution arrangement, imagecomponents having different characteristics are arranged for the regionsof the monitor frame of a distribution image. For an example, an imageis produced by mixing a text image in a region with a non-text image inanother region according to different cognitive contents. For anotherexample, an image is produced by mixing an image of high luminancecontrast in a region with an image of high specific-color contrast inanother region. If a wide disturbing image region is formed of a singledisturbing image having an image characteristic, a situation, in whichmasking is insufficient, occurs. For example, if a wide disturbing imageregion formed of text having white-black luminance contrast exists and aprivate image, such as a color photograph, is displayed in the region,masking is insufficiently performed.

FIG. 25 a illustrates an embodiment in which a disturbing image isproduced and generated based on the spatial distribution arrangement. Inthe above-described embodiment, a text image and a photograph or pictureimage having luminance or color contrast are arranged while beingspatially distributed. In the spatial overlapping arrangement, imagecomponents having different characteristics are arranged in a singleregion while overlapping each other. FIG. 25 b illustrates an embodimentin which a disturbing image is constructed using the spatial overlappingarrangement. FIG. 25 c illustrates an embodiment in which a disturbingimage is constructed using the spatial distribution arrangement and thespatial overlapping arrangement. During actual operation, the componentsor parts of the disturbing image are updated by moving or deforming themwith the lapse of time or loading new image components.

In the present invention, a disturbing image preferably includes imagecomponents having characteristics similar to those of a private image.In an embodiment, a disturbing image includes image components havingcharacteristics similar to those of a private image and image componentshaving characteristics different from those of the private image. Ingeneral, the image components having similar characteristics play achief role in releasing the cognitive grouping of a private image andthe image components having different characteristics play a chief rolein generating cognitive grouping different from that of the privateimage. If the private image is a text image, the disturbing imageincludes at least a text image component. If the private image is aphotograph image, the disturbing image includes at least a non-textimage component.

In another embodiment, masking images are more frequently displayed withthe frame ratio of a disturbing masking image being made higher than theframe ratio of a private image. In still another embodiment, a method ofcompressing the image values (Ga value) of a computer domain isutilized. In this method, the range of the Ga values (RGB values) of anoriginal image (private image) is compressed. For example, the range ofa masking image is made greater than the range of a private image as,when the masking image has a range of 0≦Ga≦255, an original image ismade to have a range of 0≦Ga≦255. In this Ga value compression method iseffective in equivalently making the frame ratio of a disturbing maskingimage higher than the frame ratio of a private image. Preferably, thecompression of Ga values is implemented by color table compression.

In an embodiment of the present invention, an entire private displaymanagement server transmits an advertisement image to a private displayuser computer through a network, and the transmitted advertisement imageis used as a disturbing image that is one of masking images.

[P/M Image Sequence and Shutter Opening/Closing Sequence GeneratingProcess]

After the user has undergone user authentication and selected a securityperformance level, a P/M image sequence and a shutter opening/closingsequence are generated according to the user authentication level andthe display security performance level. A process of generating the P/Mimage sequence and the shutter opening/closing sequence is illustratedin FIG. 26. After the user authentication, the user selects one ofpreset display security performance levels at step 2602. Thereafter, aP/M image mixing ratio and a P/M image mixing rule fulfilling thedisplay security performance level and user visual perceptionperformance are selected at step 2604. Thereafter, the P/M imagesequence is generated according to the mixing rule at step 2606.Thereafter, the shutter opening/closing sequence fulfilling the selectedsecurity performance level and the user visual perception andcorresponding to the P/M image sequence is generated at step 2608.Thereafter, the step 2610 of determining whether a desired shutteropening/closing sequence has been generated is performed. If the desiredshutter opening/closing sequence has been generated, the process ends;otherwise the process proceeds to step 2612. At step 2612, it isdetermined whether the mixing rule will be re-selected. If it isdetermined that the mixing rule will not be re-selected, the processproceeds to step 2606 in which the P/M image sequence is generated andthe following steps are performed. If it is determined that the mixingrule will be re-selected, the process proceeds to step 2604 in which theP/M image mixing ratio and the mixing rule are re-selected and thefollowing steps are performed.

[Mixing Ratio and Mixing Rule]

The security levels of step 2602 may be variously defined. For anexample, at a first level, an unauthorized person cannot perceive theapproximate type of user private images even though the unauthorizedperson views a monitor for a period longer than a certain period. Thefirst security level is the strictest private information protectionlevel. For example, at this level, the unauthorized person cannot learnwhether the user performs word processing or views moving images. At asecond level, if the unauthorized person views a monitor for a periodlonger than a certain period, the user perceives the approximate type ofuser images. However, the unauthorized person cannot learn even a partof the contents of user image information. For example, the unauthorizedperson can learn that the user is viewing moving images, but cannotlearn that the moving images are a movie or image chatting. At a thirdlevel, if an unauthorized person views a monitor for a period longerthan a certain period, the unauthorized person can approximately learn apart of the contents of user image information. However, theunauthorized person can learn most of the contents of user imageinformation. For example, the authorized person cannot learn thecontents being word-processed. The unauthorized person can learn thatthe user is viewing the moving images of a movie, but cannot learn thecontents of the moving images. At a fourth level, if an authorizedperson views a monitor for a period longer than a certain period, theunauthorized person can accurately learn a part of the contents of userimage information. However, it cannot be learned that most of thecontents of user image information. For example, some of the contentsword-processed can be learned. At a fifth level, an unauthorized personcan learn a considerable part of the contents of user image information.However, the unauthorized person senses inconvenience to visuallyperceive an image. For another embodiment, the extent to which a userprivate image and an intentional disturbing image are perceived by anunauthorized person may be added to such a performance level as anadditional performance index.

At step 2604, the P/M image mixing ratio and the mixing rule areselected. At the P/M image mixing rule selection step 2604, theselection of the type of masking images M to be mixed, the selection ofthe mean mixing ration of P/M images, and the selection of P/M imagesequence generation methodology to insert aperiodicity while maintainingthe mean mixing ratio are executed. For the image sequence generationmethodology, there are a maximum repetition period sequence method and amaximum identical characteristic continuous image sequence method thatwill be described later. In the conventional method, the P/M imagemixing rule selection step and the P/M image sequence generation stepare not divided from each other. Since in the conventional method,security performance and visual perception performance areinsufficiently taken into consideration, an image sequence is generatedin a 1:m repetition sequence or random sequence in a simple fashion. Atthe time of selecting the P/M image mixing rule, the current refreshrate of a monitor, the response time of a monitor pixel, and the opticalresponse characteristics of a shutter are all taken into consideration.Furthermore, attention should be paid to the characteristics (inverseimage or disturbing image) of P/M images. The P/M image sequence methodis ineffective in terms of various performance aspects because theabove-described four points are not taken into consideration at the timeof generating a sequence. Furthermore, at the time of generating the P/Mimage sequence, the shutter opening/closing sequence should be takeninto consideration, and even at the time of generating the shutter, theP/M image sequence should be taken into consideration.

In the conventional image mixing, private images are mixed with maskingimages having the same characteristics. That is, masking images havingthe same characteristics, such as masking images composed of inverseimages or random images, are mixed with private images. A P/M imagesequence is generated under the mixing rule. In the P/M image mixingratio and mixing rule of the present invention, images having differentcharacteristics can be mixed with each other.

For an embodiment, masking images in which original image-derived imagesare mixed with disturbing images are mixed with private images. Formethods of mixing original image-derived images with disturbing images,there are a method of mixing image components having differentcharacteristics, such as an original image-derived image and adisturbing image, with each other in a single masking image frame, and amethod of generating a masking frame M^(i) chiefly composed of originalimage-derived images and a masking frame M^(d) in which disturbingimages are added or dominant and mixing the masking frames with privateimages on a frame level. In this case, the masking frame M^(i)represents a pure original image-derived image or a sum image in whichoriginal image-derived images are dominant compared to disturbingimages, and the masking frame M^(d) represents disturbing images or thesum image of an original image-derived image and a disturbing image inwhich the portion of the disturbing image is larger than a certainvalue. The mean mixing ratio may be determined in various fashions, suchas P:M=5:5, P:M=4:6, and P:M^(i):M^(d)=4:4:2. When images are mixed witheach other at a ratio of P:M^(i):M^(d), M^(i) is made a mixed image inwhich a random image having a certain amplitude is added to an inverseimage and M^(d) is made an image frame having the characteristics of adisturbing image in an embodiment. For another embodiment, images aremixed with each other at a ratio, such as P:M^(i):M^(b) orP:M^(b):M^(d). For an example, images may be mixed with each other at aratio of P:M^(i):M^(d):M^(b)=4:4:1.5:0.5 (in this case, a type ofP:M=4:6. For another embodiment, private images may be mixed with twotypes of disturbing images as private images are mixed with disturbingimages hindering the grouping of the private images and disturbingimages grouped with each other.

For an embodiment of the present invention, a real-time originalimage-derived image is generated by randomly changing a color tableaccording to a conversion rule using the color table (see 402 in FIG.4), which is included in a video control device, such as a video card,as an image value converter. In another embodiment of the presentinvention, real-time image value conversion is implemented using aninverter or differentiating or integrating circuit.

Using the color table changing method, it is convenient to generate amasking image, in which an original image-derived image is mixed with adisturbing image, in real time. In the method of mixing an originalimage-derived image with a disturbing image in a single masking imageframe, the mixed image of an original image-derived image and adisturbing image can be generated using the color table changing methodin an embodiment. For another embodiment, a mixed image can be generatedby generating an original image-derived image using the color tablechanging method and adding a disturbing image, which is obtained throughthe calculation of pixel image values for some pixels. For anotherembodiment, the sum image of a private image and a disturbing image isgenerated by copying the private image to a masking memory space andadding a disturbing image to the copied private image. If the colortable changing method is used at the time of transmitting a sum image toa monitor, a sum image-derived converted image is transmitted, so thatthe sum image of an original image-derived image and a disturbing imageis generated and transmitted to a monitor. In a method in which maskingframes M^(i) and M^(d) are generated and mixed with private images on aframe level, when the color table changing method is used, it isconvenient to generate a masking image in real time.

For an embodiment, in the case where images are mixed at a ratio ofP:M^(i):M^(d), if M^(i) is generated from a private image in real timeusing the color table changing method and M^(d) is relatively slowlyupdated through the calculation of pixel image values, a real-time mixedmasking image can be provided as a whole. For another embodiment, in thecase where images are mixed at a ratio of P:M^(i):M^(d), M^(i) isquickly generated in real time using the color table changing method,and M^(d) is generated by relatively slowly copying private imagesseveral times per second to a masking image memory space and addingdisturbing images to the private images. When M^(d) is transmitted to amonitor, the color table changing method can be used as for M^(i). Inthis case, M^(d) can effectively mask rapidly changing private imagesand M^(i) can effectively mask slowly changing private images, so thatthe ration of M^(i):M^(d) is differently adjusted according to thecharacteristics of private images in an embodiment. For an example, whenmany still images exist in private images as in text work, the portionof M^(d) is increased. Furthermore, when many moving images exist inprivate images, the portion of M^(i) is increased. The adjustment of theratio can be executed in such a way that private display software learnsthe characteristics of a current private image and automatically adjuststhe ratio to be suitable for the learned characteristics, or the useradjusts the ratio. Furthermore, the range of the adjustment of the ratiois determined by restricting display frequencies per second for M^(i)and M^(d) in view of the current refresh rate of a monitor and thesensitive frequency characteristics of human visual perception.

In general, a masking image is made to be seen better than a privateimage, so that a mixing rule, which reduces the contrast of a privateimage compared to the contrast of a masking image, may be selected. Thecontrast of an image may be reduced through the calculation ofrespective pixel image values that consume excessive time incalculation. The reduction of the contrast of an image using conversionis termed a color space dynamic range reduction method. In this case,the term dynamic range is the concept similar to the maximum contrast ofa color space (difference between the maximum value and the minimumvalue), the range of the brightness of a monitor or the range of voltageoutput to a monitor. An embodiment is a HW type, in which the range ofvoltage of a monitor can be reduced in a hardware fashion only when aprivate image is output. Another embodiment is a HW type, in which thevoltage of a monitor is output without change when a private image isoutput, and the voltage of the monitor can be amplified when a maskingimage is output. In an SW type, for an embodiment, there is thereduction of a color space dynamic range using the color table changingmethod. For example, an input RGB range [0, 255] is reduced to an outputRGB range [56, 255].

In a public display, since the user views only a private image that is apart of a monitor image, the user may view a screen in a situationdarker than a general mode, so that a need for increasing brightnessoverall occurs. For this purpose, voltage applied to a monitor may beamplified or a color table changing value may be made to upwardlytransit to a brighter side. A method of improving the relativebrightness of a monitor using the privacy filter or brightnessenhancement film is utilized. In this process, the user's adjustmentusing the brightness adjusting button and contrast adjusting button of amonitor should be considered.

When a dynamic inverse image is generated and alternated with a privateimage, an almost uniform gray screen is seen by the naked eye. However,time integration is not complete, so that a problem arises in that auniform gray screen is displayed as a background even though a minutecolor or spatial pattern remains, and the uniform gray screen is seen.In particular, this phenomenon is conspicuous at intermediate to hightemporal frequencies (tf1˜tf3) and intermediate to high spatialfrequencies (sf1˜sf3). In an embodiment to solve the above-describedproblem, a colored dynamic inverse image is provided. The coloreddynamic inverse image is the inverse image, which is obtained byincreasing or reducing the Ga value of a specific color in a dynamicinverse image so that the colored dynamic inverse image is tinged withsomewhat colored gray when being alternated with a private image. In anembodiment, if the color of a colored dynamic inverse image is changedmany times for a single monitor frame period, for which a masking imageis displayed, using the color table changing method, many pieces ofdifferently colored inverse images can be generated in a single maskingimage monitor frame. Such a colored dynamic inverse image is effectivein blocking a minute color, space pattern due to incomplete timeintegration. In a more general embodiment, a masking image can begenerated as an image which is a dynamic inverse image in terms ofluminance and the color of which is randomly changed.

In another embodiment of the present invention, the following privateimage separation method can be used at a specific sequence position toperform smooth shutter opening/closing. For example, in the case where aprivate image P1 having a pixel image value of (R1, G1, B1) and privateimage P2 having a pixel image value of (R2, G2, B2) are continuouslydisplayed, private image P3 having a pixel image value of (R3, G3, B3)and private image P4 having a pixel image value of (R4, G4, B4) arecontinuously displayed instead. In this case, the dynamic mean pixelimage value of the private image P1 having a pixel image value of (R1,G1, B1) and the private image P2 having a pixel image value of (R2, G2,B2) is made to be identical with the dynamic mean pixel image value ofthe private image P3 having a pixel image value of (R3, G3, B3) and theprivate image P4 having a pixel image value of (R4, G4, B4).

When private display is simultaneously provided to two or moreauthorized users using a single monitor, a corresponding mixing rule isfollowed. The private image frame of user a, the masking image frame ofuser a, the private image frame of user b, and the masking image frameof user b are defined as P_a, M_a, P_b, and M_b, respectively. In thiscase, P_b and M_b as well as M_a function as masking images for the usera. When private display is simultaneously provided to two or moreauthorized users using a single monitor, images can be mixed with eachother in such a way that P_a, P_b, M_a and M_b are used as imagecomponents and the above-described mixing rules are simply modified andapplied.

The P/M image sequence generation step 2606 based on the mixing rules isdescribed in detail below. After the mixing rule of image frames isdetermined, a P/M image sequence is generated according to the mixingrule.

In Sun Microsystems' scheme, a 1:m (m=1, 2, . . . ) repetitivealternation sequence method is a convenient and efficient method if apeeper does not exist, but is weak to a peeper because a sequence has arepetitive period. In contrast, although the IBM's scheme is anasynchronous type scheme, an image frame repeatedly alternates between Pand M images, so that the IBM's scheme has an ‘anti-peeper securityperformance’ better than that of Sun Microsystems' scheme, but is stillinefficient. In MERL (Mishubishi Electric Research Lab.) scheme, a P/Mimage frame sequence is randomly generated and provided to improve‘anti-peeper security performance.’ When the P/M image frame sequence israndomly generated, ‘anti-peeper security performance’ is improved andthe difference between the duration of shutter opening and the durationof shutter closing becomes irregular, so that it is considerablyinconvenient and fatigued in terms of visual perception for anauthorized user to view images.

In the present invention, a maximum repetitive period sequence method isproposed to overcome problems attributable to the regular repetitiveperiodicity of the Sun Microsystems' scheme and the IBM's scheme andproblems attributable to the random irregularity of the MERL's scheme.Whether a P/M image sequence is repetitive is determined depending onwhether a specific region of a monitor is sequentially repetitive on thebasis of a monitor frame. Accordingly, whether a P/M image sequence isrepetitive may vary with the regions of a monitor. Although the P/Mimage sequence method of the IBM's scheme is an asynchronous typemethod, a P image is displayed in the k-th frame, and a P image isdisplayed up to a specific region of the upper end of the (k+1)th frameand an M image is displayed in the remaining region of the (k+1)thframe, so that P images are repetitively displayed in the kth monitorframe, the (k+2)th monitor frame, the (k+4)th monitor frame, . . . .That is, an image temporally alternates between a P image and an M imageon the basis of a monitor frame, so that the characteristic of arepetitive sequence is exhibited and ‘anti-peeper security performance’is reduced.

The maximum repetitive period sequence method is the method in which themaximum allowable number of sequence repetitive periods is limited sothat there is generated the sequence in which aperiodicity, such as aphase change, is inserted into a regularly repetitive period sequence.The sequence method regularly inserts aperiodicity, such as a phasechange, into repetitive period sequences to fulfill ‘anti-peepersecurity performance,’ ‘user visual perception performance’ and ‘nakedeye security performance.’ In general, a peeper easily interpret animage sequence having a repetitive period, particularly a P/M imagesequence in which a private image has a regular repetitive period. Inthe present invention, when the maximum allowable number of sequencerepetitive periods is m, all image sequences are each generated to havem or less repetitive periods. In an embodiment of the maximum repetitiveperiod sequence method applied to a vertical sync type in which P/Mimages are alternated in accordance with the vertical sync of a monitor,aperiodicity is inserted at a certain moment while a repetitive sequenceprogresses as shown in FIG. 27. For an embodiment, when a monitor framehas a PM repetitive period as in . . . PMPMPMPMPM . . . , a sequencehaving the maximum number of repetitive periods, m=4, does not allowfive-consecutive repetitive periods as in . . . PMPMPMPMPM . . . , sothat aperiodicity, such as a sequence phase change, is inserted as in .. . PMPMPMPMMPMP . . . or . . . PMPMPMPMMMPM . . . . Furthermore, thesequence method of the present invention is the sequence generationmethod in which repetitive periods, the number of which is equal to orless than the maximum allowable number of repetitive periods, arearranged in random order. For an embodiment, when a PM repetitive periodexists and m=4, there is generated a sequence in which afour-consecutive repetitive period unit, phase change aperiodicity, athree-consecutive repetitive period unit, and a two-consecutiverepetitive period unit are arranged in random order or randomly. Forexample, the image sequence in which a monitor frame is generated as in. . . PMPMPMPMMPMPMPMMPMPMP . . . is the sequence that is arranged inthe order of a four-consecutive repetitive period unit, phase changeaperiodicity, a three-consecutive repetitive period unit, phase changeaperiodicity and a three-consecutive repetitive period unit to have aunit component of . . . (PMPMPMPM) (MPMPMP)M(MPMPMP).

The repetitive period of a monitor frame may has various forms, such asPMM, PPM, PPMM and PMPMM, besides a PM repetitive period. For anembodiment, when a PMM repetitive period exists as in . . . PMMPMMPMM .. . and m=3, phase change aperiodicity is inserted afterthree-consecutive repetitive periods as in . . . (PMMPMMPMM) (PMPMM) . .. or . . . (PMMPMMPMM) (MPMM) . . . .

In another embodiment of the present invention, a P/M image sequence, inwhich the maximum number of repetitive periods varies with time, isgenerated. That is, the maximum number of repetitive periods varies withtime in such a way that the maximum number of repetitive periods is m1for a specific time period and the maximum number of repetitive periodsis m2 for a next specific time period. When the maximum number ofrepetitive periods is mi for a specific time period, a sequence isgenerated for the specific time period according to the maximumrepetitive period sequence method with mi being used as the maximumnumber of repetitive periods. A P/M image sequence, in which the form ofa unit repetitive period varies with time by optionally or randomlyvarying the maximum number of repetitive periods with time or optionallyor randomly varying the width of time having the same maximum number ofrepetitive periods, is generated.

In another embodiment of the present invention, an M image sequence, inwhich the maximum number of repetitive periods and the form of a unitrepetitive period vary with time. In another embodiment of the maximumrepetitive period sequence method, a sequence, in which one or moreoptional or random sequences are added to or inserted into a sequencegenerated by the maximum repetitive period sequence method, isgenerated.

In an embodiment of the maximum repetitive period sequence methodapplied to an asynchronous type, aperiodicity is inserted at a certainmoment while a repetitive sequence progresses as shown in FIG. 28. Whenthe maximum repetitive period sequence method is applied to anasynchronous type, an image data cycle time should be selected dependingon the refresh cycle time of a monitor frame. If the image data cycletime is not appropriately selected, a specific region of the upper orlower end of a monitor is periodically exposed and the probability thata sequence will be interpreted by a peeper is increased even thoughaperiodicity is inserted.

When a P/M image sequence is generated by inserting aperiodicity, ‘uservisual perception performance’ should be consisderd. Human visualperception is sensitive to a brightness variation within a temporalfrequency range of about 10˜30 Hz. Accordingly, aperiodicity in atemporal frequency range of about 10˜30 Hz is inserted, the user willhave visual perception in which the aperiodicity has been inserted. Theuser must perceive a private image without external disturbance, so thatthe perception of the flow of an image sequence by user's eye interfereswith the comfortable perception of the private image and may cause theeye to be strained. In order to reduce the discomfort of visualperception, a P/M image sequence, in which phase change aperiodicity isinserted 15 or less times per second, is generated in an embodiment ofthe present invention. In another embodiment, a P/M image sequence, inwhich phase change aperiodicity is inserted 25 or less times per second,is generated.

In an embodiment of the present invention, ‘anti-peeper securityperformance’ can be further improved by operating a monitor at variousrefresh rates in addition to the maximum repetitive period sequencemethod. That is, even though not supported in a VESA standard monitor, amonitor can be operated at refresh rates, such as 101 Hz and 103 Hz.

Meanwhile, if images having the same characteristics, such as PPP orMMM, are consecutively displayed in the monitor frames of vertical synctype private display, a peeper can easily learn the sequence, and thedisplaying of the images causes excessive contrast, thus fatiguing theeye. The maximum same characteristic consecutive image sequence methodof the present invention proposed to solve the above problem is themethod of restricting the maximum allowable number of images having thesame characteristics that can be consecutively displayed in a P/M imagesequence. In an embodiment, it is determined whether the samecharacteristics exist in images, depending on whether the images are Por M images. In another embodiment, it is determined in detail whetherthe same characteristics exist in M images, depending on whether the Mimages have the same masking characteristics. In an embodiment, when itis determined whether the same characteristics exist in images,depending on whether the images are P or M images, the maximum samecharacteristic consecutive image sequence method that allows a maximumof m consecutive image frames having the same characteristics is asfollows.

A method, which allows a maximum of 1 image frame having the samecharacteristics (prohibits two-consecutive images having the samecharacteristics), generates 1:1 alternation images, such as PMPMPM . . ., which has no ‘anti-peeper security performance.’ A method, whichallows a maximum of 2-consecutive images having the same characteristics(prohibits 3-consecutive image frames having the same characteristics),generates an image sequence, such as PPMMPMMPPM . . . . The method ofallowing 2-consecutive images having the same characteristics randomlygenerates first and second images. If the (k−1)th frame is differentfrom the (k−2)th frame at the time of generating the kth frame, the kthframe is formed of an image having optional or random characteristics.If the (k−1)th frame is not different from the (k−2)th frame, the kthframe is formed of an image having characteristics different from thoseof the (k−1)th frame. For example, if (k−1)th frame is a P image, thekth frame is formed of an M image.

Furthermore, the maximum same characteristic consecutive image sequencemethod of the present invention generates a P/M image sequence in whicha maximum allowable value varies with time. That is, the maximumallowable value varies with time in such a way that the maximumallowable number of consecutive images having the same characteristicsfor a specific time period is m1 and the maximum allowable number ofconsecutive images having the same characteristics for a next specifictime period is m2. If the maximum allowable value is mi, a sequence isgenerated for a specific time period according to the maximum allowablenumber sequence method with mi being used as the maximum allowablevalue. A P/M image sequence, in which the maximum allowable value varieswith time by optionally or randomly varying the maximum allowable numberwith time or optionally or randomly varying the width of time having thesame maximum number, is generated.

In another embodiment of the maximum same characteristic consecutiveimage sequence method, a sequence, in which one or more optional orrandom sequences are added to or inserted into a sequence generated bythe maximum same characteristic consecutive image sequence method, isgenerated. The maximum same characteristic consecutive image sequencemethod is the method that improves ‘user visual perception performance’by reducing the strain of the eye. With this method, a sequence, inwhich P and M images are simply and repetitively alternated with eachother, such as PMPMPM . . . , is generated, so that this method is notthe method in which ‘anti-peeper security performance’ is taken intoconsideration. In another embodiment of the present invention, a P/Mimage sequence is generated under the methodology in which the maximumrepetitive period sequence method and the maximum same characteristicconsecutive image sequence method are integrated with each other.

In another embodiment of the present invention, the same characteristicconsecutive image method is performed with it is determined whether thesame characteristics exist, depending on whether the M images have thesame masking characteristics. In this case, when masking images aredivided into M^(i) and M^(d) and identified as frames having differentcharacteristics, PM^(i)M^(d)PM^(i)M^(d) is a sequence having1-consecutive images. Even though the maximum same characteristicconsecutive image sequence method is performed while determining whetherthe same characteristics exist in detail, this maximum samecharacteristic consecutive image sequence method is performed in thesame process as the method in which it is determined whether the samecharacteristics exist in images, depending on whether the images are Por M images.

If a P/M image sequence is generated as described above, a shutteropening/closing sequence fulfilling a selected security performancelevel and user visual performance and corresponding to the P/M imagesequence is generated. FIGS. 29 to 34 are drawings illustrating aprivate image and masking image sequence and a shutter opening/closingsequence in accordance with the present invention.

A conventional shutter opening/closing sequence operates a shutter intwo states, including opening and closing. When a private image isdisplayed according to a P/M image sequence, the shutter is opened. Whena masking image is displayed, the shutter is closed. Since the P/M imagesequence and the shutter opening/closing sequence are operatedcorrespondingly in the same fashion as described above, it is notnecessary to take the P/M image sequence and the shutter opening/closingsequence into consideration independently.

If an optical signal having a different time phase is incident on theeye while temporally periodic optical signals are incident on the eye,visual perception sensitively reacts to the optical signal having adifferent time phase due to the temporal frequency sensitivity andtemporal grouping of human visual characteristics. If a P/M imagesequence is repetitively and periodically alternated to allow theoptical signal of an image to be incident on the user's eye, and ashutter is repetitively and periodically opened and closed, it isdifficult for the user to perceive the opening/closing of the shutterand the alternation of P and M images and the user consecutivelyperceives private images, so that it is comfortable to view the sequenceand visual strain becomes low. However, when a P/M image sequencegenerated to include phase change aperiodicity so as to improve‘anti-peeper security performance’ is viewed in a conventional shutteropening/closing fashion, visual perception sensitively reacts to a phasechange aperiodicity component, so that it is inconvenient to view thesequence and strain occurs. In particular, visual perception is moresensitive to the phase change aperiodicity component of a shutteropening/closing means. In an embodiment of the present invention, theabove-described problem is solved by generating a P/M image sequence, inwhich aperiodicity is inserted 15 or less times or 25 or less times persecond, and generating a shutter opening/closing sequence to correspondto the P/M image sequence.

Meanwhile, when a P/M image sequence is generated with aperiodicitybeing inserted, ‘user visual perception performance’ depends on thecontrast of the quantity of light except for a sequence frequency. Thequantity of light incident on the user's eye includes ambient lightexcept for the light of the images of a monitor. In particular, thestructure of human visual perception is sensitive to the variation ofbrightness of ambient light rather than central light. For this reason,in the place where surroundings around a monitor are dark, the contrastof the quantity of light of shutter opening/closing is not high, so thatit is difficult for the user to perceive the flow of an image sequence.In contrast, in the place where surroundings around a monitor arebright, the contrast of the quantity of light of shutter opening/closingis increased, so that it becomes easy for the user to perceive the flowof an image sequence. Accordingly, when the private display is utilizedin a bright place, it is easy due to the high contrast of the quantityof light for the user to feel fatigue.

In order to solve the problem of the contrast of the quantity of light,the present invention proposes an intermediate state shutteropening/closing method. A shutter is opened and closed in multiplestates in such a way that an intermediate state is set between twostates of opening and closing. If, at the time of opening/closing ashutter, the relative light transmittance of a maximum opening state isnormalized to 1, the relative light transmittance of a maximum closingstate is normalized to 0 and the relative light transmittance of anintermediate state is set to a value between 0 and 1 according to theopening/closing light transmittance, the shutter is opened and closed inthe intermediate state. If voltage corresponding to shutteropening/closing relative light transmittance between 0 and 1 is appliedto a shutter, the shutter opening and closing light transmittance can becontrolled, thus producing intermediate state shutter opening andclosing. In most of shutters, the shutters are in a maximum openingstate when voltage is not applied to the shutters, and lighttransmittance is further decreased as voltage is applied to the shuttersmore.

In particular, the intermediate shutter opening/closing method isapplied to the location of a P/M image sequence where phase changeaperiodicity exists or the vicinity thereof. A vertical sync typeembodiment of a process of generating a shutter opening/closing sequencecorresponding to a P/M image sequence using the intermediate stateshutter opening/closing method is illustrated in FIGS. 29 to 34. FIG. 29illustrates an embodiment in which phase change aperiodicity is insertedat the fourth location of an image sequence, in which images arealternated to have a repetitive period of PM, and P images are generatedat the third and fourth locations. In this case, for an example,intermediate state shutter opening/closing are performed at the thirdand fourth locations, as shown in FIG. 29. The relative lighttransmittances T of the third and fourth location are T(3) and T(4),respectively, and voltage to be input to a shutter is controlled toallow T(3) and T(4) to have values between 0 to 1. The inconvenience ofvisual perception due to phase change aperiodicity is influenced by theduration of the phase change aperiodicity and contrast. Intermediateshutter opening/closing is performed in view of the contrast of ashutter light transmittance sequence and the contrast of a P/M imagesequence at locations where P images are arranged due to phase changeaperiodicity, as indicated by T(3) and T(4). In another embodimenthaving such a situation, when 2-consecutive P images are displayed, thecontrast of a P image can be reduced using the color space dynamic rangereduction method.

FIG. 30 illustrates an embodiment in which phase change aperiodicity isinserted at the fifth location of an image sequence, in which images arealternated using a repetitive period of PM, and M images areconsecutively generated at the fourth and fifth locations. In this case,for an example, intermediate state shutter opening/closing is performedat the third and fourth locations. The Intermediate shutteropening/closing is performed in view of the contrast of a shutter lighttransmittance sequence and the contrast of a P/M image sequence in thevicinity of locations where two consecutive P images are arranged due tophase change aperiodicity, as indicated by T(3) and T(4). A shutter isopened in the intermediate state in response to an M image, so the userviews a masking image, and thus a problem arises in that visualperception is inconvenient. Even though the M image corresponds to theintermediate state shutter opening/closing is a general original imagederived image or disturbing image, there is no considerableinconvenience of visual perception. However, to eliminate theinconvenience of visual perception, the M image is generated as aconnection image frame M^(b) in an embodiment. The connecting imageframe is the image frame that is inserted to perform smooth shutteropening and closing or smooth image alternation, as which a blank imageframe, uniform gray image frame or uniform colored image frame is used.

FIG. 31 illustrates an embodiment in which phase change aperiodicity isinserted at the fifth location of an image sequence, in which images arealternated using a repetitive period of PM, and M images areconsecutively generated at the fourth and fifth locations. In this case,for an example, intermediate state shutter opening/closing is performedonly at the fourth location. FIG. 32 illustrates an embodiment in whicha PM sequence, which is phase change aperiodicity, is inserted at thefourth and fifth location of an image sequence in which images arealternated using a repetitive period of PMM. For an embodiment of the Mimage corresponding to the intermediate state shutter opening/closing,the M image is generated as a connecting image frame M^(b). FIGS. 33 and34 illustrate other embodiments that show intermediate state shutteropening/closing sequences that continuously vary the light transmittanceof a shutter during a single image frame period with respect to a P/Mimage sequences identical with that of FIGS. 31 and 32. Shutteropening/closing sequences shown in FIGS. 29 to 34 are ideal lighttransmittance waveform sequences. A shutter actually implemented withliquid crystal deviates from an ideal light transmission waveform due torising time and falling time attributable to light response time.

FIGS. 35 and 36 are drawings illustrating the relation between shutterrelative light transmittance and shutter opening/closing sequence statevalues. FIG. 35 illustrates the relation between the relative lighttransmittance of a shutter and voltage input to the shutter. FIG. 36illustrates the relation between voltage input to the shutter andshutter opening/closing sequence state values. FIG. 35 illustrates thecase where a shutter is in a maximum opening state (1) when voltage isnot applied and the shutter is in a maximum closing state (0) whenmaximum voltage Vmax is applied, as in a liquid crystal shutter. For anembodiment, when shutter opening/closing signals indicate two shutterstates of opening and closing, the shutter opening/closing sequencestate value of a maximum opening state (1) is set to “0”, and theshutter opening/closing sequence state value of a maximum closing state(0) is set to “1.”

In another embodiment, when shutter opening/closing signals indicatemultistage shutter states, including an intermediate state, shutteropening/closing sequence state values corresponding to shutterintermediate states are appropriately calculated with the shutteropening/closing sequence state value of a maximum opening state (1) setto “0” and the shutter opening/closing state value of a maximum closingstate (2) set to “Dmax.” For example, intermediate state values arescaled in 8 bits, Dmax=255 and the intermediate state shutteropening/closing sequence state values have values between 0˜255.

A shutter controller applying voltage to a shutter receives shutteropening/closing sequence state values corresponding to theopening/closing of a shutter and the extent of the opening/closing ofthe shutter, which are included in a shutter opening/closing signal, andcontrols the shutter. The intermediate state shutter opening/closing canbe implemented by a modulation method, such as Pulse Width Modulation(PWM), or the application of Direct Current (DC) voltage in the shuttercontroller. For an embodiment, when the intermediate shutteropening/closing is implemented by the application of DC voltage, theshutter controller converts received shutter opening/closing statevalues into analog voltages and applies the analog voltages to ashutter. For example, when a shutter is operates in a range of 0˜12 V,the minimum value of a shutter opening/closing sequence state value “0”is converted into 0 V, the maximum value thereof “Dmax” is convertedinto 12 V and the 0 V and 12 V are applied.

In an embodiment in which the intermediate shutter opening/closing isimplemented by PWM, for example, when a shutter is operated in a rangeof 0˜12 V, the extent of the opening/closing of the shutter isdetermined depending on the ratio of a shutter closing interval (12 V)to a shutter opening interval (0 V). When a shutter is operated at +/−12V and 0 V using PWM, the extent of the opening/closing of the shutter isdetermined depending on the ratio of a shutter closing interval (12 V)to a shutter opening interval (0 V). The above case employs thecharacteristics of liquid crystal in which a liquid crystal shutter isclosed when 12 V or −12 V is applied to liquid crystal and the liquidcrystal is in a light transmitting state when 0 V is applied to theliquid crystal. Since the mean light transmittance of a shutter becomeslower as the shutter closing interval becomes larger relative to theshutter opening interval, the shutter controller determines the ratio ofa shutter closing interval to a shutter opening interval in proportionto received shutter opening/closing sequence state values and operatesthe shutter using PWM. Many ripples occur in the light transmissionwaveform of a shutter intermediate state using PWM. If a monitor isviewed in this state, the non-uniformity of an image may be caused bythe ripples. Accordingly, in the shutter intermediate state implementedusing PWM, the non-uniformity of an image is low when a correspondingimage frame is a connecting image frame, such as a blank image, which iseffective.

A display, such as an LCD, is problematic in that an afterimage remainsbecause the light response time of liquid crystal that is a monitorpixel. The schematic response curve of the light responsecharacteristics of a monitor pixel is illustrated in FIGS. 37 a and 37b. The shutter opening/closing sequences of the present inventioncompensating for the slow response time of a monitor pixel areillustrated in FIGS. 37 a and 37 b. The image frame screens of FIGS. 29to 34 each and integrally illustrate images displayed for a singlemonitor frame period, whereas the P/M image frame screens of each ofFIGS. 38 to 40 are images captured at specific points within a singlemonitor frame period. When a single monitor frame period is ts, each ofFIGS. 38 to 40 illustrate images captured when 0.2 ts, 0.5 ts, 0.7 tsand 0.9 ts have elapsed after the vertical sync of a monitor frame,respectively. The screens for a monitor in which the light response timeof a monitor pixel, such as a CRT, are as shown in FIG. 38. In the casewhere a P image is displayed after an M image has been displayed, asshown in the third screen of FIG. 38, a P image starts to be displayedon the upper portion of a monitor at 0.2 ts after the start verticalsync of P image display, and the remaining portion of the screen “b” isblank. At 0.5 ts after the vertical sync, a P image starts to bedisplayed on the center portion of the monitor, and the P imagedisplayed on the upper portion disappears and is changed to a blankimage b. For such a monitor having such a quick response time, a simpleshutter opening/closing sequence switching a maximum opening state and amaximum closing state in accordance with vertical sync, as shown in thedrawing, is effective.

In contrast, screens for a monitor having the somewhat slow lightresponse time of a monitor pixel, such as a LCD monitor, are illustratedin FIG. 39. This is the embodiment in which it is assumed that theduration of an afterimage is shorter than a single monitor frame period.In the case where a P image is displayed after an M image has beendisplayed, as shown in the third screen, at 0.2 ts after the startvertical sync of P image display, a P image starts to be displayed onthe upper portion of a monitor, the center portion thereof is black “b”and the afterimage of a previous M image remains in the lower portionthereof. At 0.6 ts after the vertical sync, a P image starts to bedisplayed on the center portion of the monitor, the P image displayed onthe upper portion remains, and a blank image b and the afterimage of theM image remain in the lower portion. At 0.7 ts after the vertical sync,a P image starts to be displayed on the lower center portion of themonitor, the P image displayed on the upper portion remains and theafterimage of the M image completely disappears and is changed to ablank image “b.” At 0.9 ts after the vertical sync, a P image starts tobe displayed on the lower portion of the monitor, and most of the screenof the monitor is filled with the P image. In the monitor having asomewhat slow response time, compensation is performed by applying arandom point intermediate state shutter opening/closing sequence, asshown in FIG. 39. As described above, a shutter opening/closing sequenceis generated using the random point intermediate state shutteropening/closing sequence in view of the light response time of themonitor pixel. The embodiment as shown in FIG. 39 is particularlyeffective for the private image of the center portion of a monitor.

The screens for a monitor having the slow light response time of amonitor pixel are as shown in FIG. 40. This is the embodiment in whichit is assumed that the duration of an afterimage is equal to or longerthan a single monitor frame period. In the case where a P image isdisplayed after an M image has been displayed, as shown in the thirdscreen of the drawing, at 0.2 ts after the start vertical sync of Pimage display, a P image starts to be displayed on the center portion ofa monitor, and the afterimage of a previous M image remains on the lowercenter portion of the monitor. At 0.7 ts after the vertical sync, a Pimage starts to be displayed on the low center portion of the monitor,the P image displayed on the upper portion remains, and the afterimageof the M image remains in the lower portion thereof. At 0.9 ts after thevertical sync, a P image starts to be displayed on the lower portion ofthe monitor, and most of the screen of the monitor is filled with the Pimage. In such a monitor having a slow response time, compensation isperformed using the random point intermediate state shutteropening/closing sequence, as shown in FIG. 40. For an embodiment, ashutter opening/closing sequence, such as T2(k) or T3(k) of FIG. 40, maybe applied. In the present invention, the P/M image sequence may not besynchronized with the shutter opening/closing sequence, like the aboveembodiment. In an embodiment of the present invention, a sequencemethod, in which the P/M image sequence is not synchronized with theshutter opening/closing sequence at an optional point, may be utilizedwithout regard to intermediate state shutter opening/closing, shutteropening/closing in two states of opening/closing or a sequence method.

In monitors having a slow response time, as shown in FIGS. 39 and 40,since it may be difficult to display image values if the differencebetween the pixel image value of a current frame and the image value ofa previous frame is large, a color space dynamic range reduction methodis utilized to reduce the contrast of an image in an embodiment. Asshown in FIGS. 39 and 40, in an embodiment of the present invention, amasking image division method is used as a method effective for themonitor having a slow response time. In general, since the scanning of amonitor is performed from top to bottom, the M images of the upper andlower ends are easily viewed by the user in the monitor having a slowresponse time when the shutter is opened, as shown in FIGS. 39 and 40.In the masking image division method of the present invention, a maskingimage having high masking power is generated in a region completelyblocked by a shutter (chiefly, center portion), and a masking imagehaving low masking power is generated in a region incompletely blockedby the shutter. For the masking image having low masking power, thereare a connecting image, an unclear image and an image not including afrequency to which human visual perception is not sensitive. The maskingimage division method is the method in which a partial region has low“naked eye security performance’ but improved ‘user visual perceptionperformance.’

Since there are many cases in which, in a monitor having a slow responsetime, ‘naked security performance’ may vary with display regions, acorresponding shutter opening/closing sequence is generated when theuser moves a display region having high ‘naked security performance’using mouse scroll in a user interface. In another embodiment, to allowa private display region to be sequentially scrolled and viewed, andthus allow an entire screen to be viewed to the user, a shutteropening/closing sequence is sequentially changed at a predeterminedrate. At this time, if the masking image division method is employed, aheavily masked region moves accordingly. In another embodiment, a LCDdriver chip may be programmed so that display is not performed from thetop, but a central region is first displayed.

[Region Division Image Arrangement Sequence]

In an embodiment of the present invention, the private display using aregion division image arrangement sequence method, in which two or moredifferent image frames are displayed for a single monitor frame period,is proposed. If a plurality of different image frames are displayed fora single monitor frame period and shutter opening/closing is effectivelyperformed, it can be made extremely difficult that a peeper interprets aprivate image.

However, due to the slow light response of a shutter, difficulty ingenerating a masking image and the problem of the difference in thedensity of light intensity, the conventional private display haslimitations in displaying two or more different frames for a singlemonitor frame period and privately viewing the frames. As a result, inthe case of a synchronous private display, only a single kind of imageframes are displayed. In the IBM's scheme, only a single kind of imagesare displayed or two kinds of image frames are restrictively displayed,which is disadvantageous in that the slow light response of a shutter iscompensated for and the difference in the density of light intensityvaries with the regions of a monitor. In particular, the probabilitythat a P frame is displayed on a specific region of a monitor (upperend) is increased, so that the specific region has the density of lightintensity higher than other regions.

If two or more different image frames are displayed for a single monitorframe and the opening/closing of a shutter are smoothly performedcorrespondingly, more improved private display can be achieved.

FIG. 41 is a view illustrating partial screen private display accordingto the present invention. One of the monitor frame region division imagearrangement sequence methods is the partial screen private displaymethod. In order to solve the slow response of a monitor pixel and theslow response of a shutter opening/closing and reduce the calculationtime of a masking image, it may be advantageous that only a part of amonitor screen is made a private display. For an example, images aredisplayed using a P/M image sequence, as shown in FIG. 41 a. The secondscreen thereof illustrates that a PMP image frame is displayed for asingle monitor frame period. The PMP image frame may be displayed insynchronization with a specific monitor horizontal sync Hsync, or may bedisplayed in synchronization with not a monitor but only a data sync. Atthis time, a shutter opening/closing sequence can be generated in twoforms, as shown in FIG. 41 a. In a left shutter opening/closingsequence, the opening and closing of a shutter are performed insynchronization with a monitor vertical sync. In a right shutteropening/closing sequence, the opening and closing of the shutter areperformed in synchronization with alternation in a P/M image sequence.However, when the shutter is opened and closed using the right shutteropening/closing sequence, a problem arises in that the density of lightintensity of the upper and lower portions of a monitor is larger thanthat of the center portion of the monitor.

In another embodiment, the masking image division method is employed. Inthe masking image division method, a masking image Ms having low maskingpower is generated on a region (chiefly, center region) completelyblocked by a shutter and a masking image Mw having low masking power isgenerated on a region incompletely blocked by the shutter. For maskingimages having low masking power, there are a connecting image, anunclear image and an image not including a frequency to which humanvisual perception is sensitive. In this case, an image frame MwMsMw isdisplayed for a single monitor frame period.

In the case where ‘naked eye security performance’ varies with displayregions, a corresponding P/M image sequence and a corresponding shutteropening/closing sequence are generated when the user moves a privatedisplay region having high ‘naked eye security performance’ using mousescroll in a user interface. In another embodiment, to allow a privatedisplay region to be sequentially scrolled and viewed, and thus allow anentire screen to be viewed to the user, a P/M image sequence and ashutter opening/closing sequence are made to be sequentially changed ata predetermined rate.

For another embodiment, images are displayed using a P/M image sequence,as shown in FIG. 41 b. The second screen thereof illustrates that a PMPimage frame is displayed for a single monitor frame period. The PMPimage frame may be displayed in synchronization with a specific monitorhorizontal sync Hsync, or may be displayed in synchronization with not amonitor but only a data sync. At this time, on the center region thereofon which an M image is displayed, a P image is displayed after a monitorhorizontal sync Hsync, an M image is displayed after a predeterminedperiod, and a P image is displayed again after a predetermined period.In this case, a shutter opening/closing sequence can be generated in twoforms, as shown in FIG. 41 b. In a left shutter opening/closingsequence, the opening and closing of a shutter are performed insynchronization with a monitor vertical sync. In a right shutteropening/closing sequence, the opening and closing of the shutter areperformed in synchronization with alternation in a P/M image sequence.However, when the shutter is opened and closed using the right shutteropening/closing sequence, a problem arises in that the density of lightintensity of the right and left portions of a monitor is larger thanthat of the center portion of the monitor. This partial screen privatedisplay can be implemented in a software (SW) or hardware (HW) fashion.

An embodiment of the private display method, in which two or moredifferent image frames are displayed for a single monitor frame period,is as shown in FIG. 42. The region division image arrangement sequencemay be displayed in synchronization with a monitor horizontal syncHsync, or may be displayed in synchronization with not a monitor but adata sync. The embodiment of FIG. 42 is an example of the sequence inwhich image frames are arranged with each of the frames divided intothree regions. In the embodiment, a P/M image sequence is displayed inthe order of (PMP), (PMP), (MPM) and (PMP). In the embodiment, a shutteropening/closing sequence is composed of two states, including openingand closing states. In a shutter opening/closing signal transmissionpulse string, a shutter opening/closing signal is transmitted when theshutter state of the shutter opening/closing sequence is changed, and ashutter opening/closing signal is additionally transmitted at a randommoment when the shutter state is not changed, as shown in FIG. 42 c. InFIG. 42 c, the shutter opening/closing signal transmitted when theshutter state is changed is represented by gray, and current shutterstate maintaining shutter opening/closing signal transmitted at a momentwhen the shutter state is not changed is represented by white. Thecurrent shutter state maintaining shutter opening/closing signal is thesignal used to prevent a peeper from interpreting the shutteropening/closing signal. In another embodiment, a shutter opening/closingsignal is transmitted only when the shutter state of the shutteropening/closing sequence is changed. In another embodiment of thepresent invention, a shutter opening/closing sequence is composed ofmultistage states including an intermediate shutter state. In FIG. 42 a,a P image display cycle time and an M image display cycle time are fixedin an embodiment, and vary in another embodiment. By varying the P imagedisplay cycle time and the M image display cycle time, “anti-peepersecurity performance’ can be improved.

In the region division image arrangement sequence method of the presentinvention, a plurality of different image frames are displayed for asingle monitor frame period, so that further various masking imageprovision methods can be employed compared to the prior art. In thepresent invention, a masking image is provided using an originalimage-derived image method, a disturbing image method and an originalimage-derived image and disturbing image mixing method. In anembodiment, masking images having different characteristics may beprovided to the divided regions of a monitor screen, respectively. Forexample, in the case of a sequence (PMP), (PMP), (MPM) and (PMP), asshown in FIG. 42, the M image of a center region can be generated usingthe original image-derived image and disturbing image mixing method andthe M images of the upper and lower regions can be generated using theoriginal image-derived image method. For an embodiment, to effectivelygenerate a masking image in real time, the M image of a specific region,such as a center region, can be quickly updated and provided, and the Mimage of another region can be slowly updated and provided. In anembodiment of the present invention, an original image-derived image,such as a dynamic inverse image, and a disturbing image are generated inreal time using the real-time color table changing method and providedto respective regions.

FIG. 43 is a view illustrating the private image grouping method of theregion division image arrangement sequence according to the presentinvention. By varying a P/M image display cycle time to improve‘anti-peeper security performance,’ the cases where the frequency of thedisplay of private images is high in a specific portion of a monitorcompared to other regions. When the user views a monitor, the density oflight intensity of the specific portion is higher than that of otherregions, so that the user may feel inconvenience. In order to solve theproblem of the difference in the density of light intensity, the presentinvention proposes the private image grouping method. The private imagegrouping method is the method in which m complete private image screensare made to be displayed in groups for n monitor frames. In anembodiment, as shown in FIG. 43 a, P/M images are mixed with each otherin a ratio of P:M=1:1 and one complete private image screen is displayedin a group for two monitor frames. When (PMP) and (MPM) screens aredisplayed in a group, a completely combined private image screen isformed and viewed to the user. At this time, the boundaries of the (PMP)screen are made to coincide with the boundaries of the (MPM) screen. Inan embodiment, as shown in FIG. 43 b, P/M images are mixed with eachother at a ratio of P:M=1:2 on the average and one completely privateimage screen is displayed in a group for three monitor frames. If(PMM)=(PM), (MPM), (MMP)=(MP) screens are displayed in a group, onecompletely combined private image screen is viewed to the user. At thistime, the region dividing image change boundaries of respective screensare made to coincide with each other, respectively. For anotherembodiment, m complete private image screens are displayed in groups forn monitor frames as two complete private image screens are displayed ingroups for 5 monitor frames. In another embodiment, at the time ofprivate image grouping, m private image screens are not completelygrouped for n monitor frames but are grouped with specific regionsexcluded therefrom. Unless the grouping is performed with the sameregion continuously excluded therefrom, there is no inconvenience ofuser visual perception.

FIG. 44 is a view illustrating the variation of private image groupingaccording to the present invention. In an embodiment of the presentinvention, the private image grouping is varied to improve ‘anti-peepersecurity performance.’ Embodiments of the variation of private imagegrouping are illustrated in FIGS. 44 a to 44 b. In an embodiment, asshown in FIG. 44 b, a grouping unit is varied in such a way that agrouping method of allowing 1 complete private image screen to bedisplayed for 2 monitor frames is varied to a grouping method ofallowing 1 complete private image screen to be displayed for 3 monitorframes. In an embodiment, as shown in FIG. 44 b, grouping is varied insuch a way that the number of divided screen regions is varied from 3 to4 while a grouping unit is fixed so that 1 complete private image screenis grouped for two monitor frames.

FIG. 45 is a view illustrating a maximum repetitive period sequencemethod for a region division image arrangement sequence. In order toimprove ‘anti-peeper security performance,’ an embodiment of the presentinvention employs the maximum repetitive period sequence method in whichaperiodicity is inserted into the region division image arrangementsequence. It is determined whether a P/M image sequence is repeated,depending on whether the respective regions of a monitor are temporallyrepetitive on the basis of a monitor frame. For an embodiment, FIG. 45 aillustrates an embodiment in which aperiodicity is inserted at the samepoint over all the regions of a monitor. Aperiodicity is inserted in afifth frame, as in (PMP), (MPM), (PMP), (MPM), (MPM), (PMP), (MPM) . . .. PMPMMPM sequence is displayed on the upper and lower regions of amonitor, and MPMPPMP sequence is displayed on the center region thereof.In another embodiment, as shown in FIG. 45 b, aperiodicity may beinserted at different points for the respective regions of a monitor. Inthis case, images are displayed, as in (PMP), (MPM), (PMP), (MPM),(MPP), (PMM), (MPM) . . . . PMPMMPM sequence is displayed on the upperportion of a monitor, MPMPPMP sequence is displayed and aperiodicity isinserted in a fifth frame on the center portion thereof, and PMPMPMMsequence is displayed and aperiodicity is inserted in a seventh frame onthe lower portion thereof. In an embodiment of the present invention, ashutter opening/closing sequence may be generated using intermediateshutter opening/closing.

FIG. 46 is a view illustrating the sequences, shutter opening/closingsequences, and shutter opening/closing sequence light response of aprivate image and a masking image. In a private display of the presentinvention, in which two or more different image frames are displayed fora single monitor frame period, the problem of the difference in thedensity of light intensity must be solved. To this end, the presentinvention proposes a method of processing the boundary region of animage frame. FIG. 46 illustrates shutter opening/closing sequence andshutter opening/closing sequence light response according to a P/M imagesequence. An ideally fast shutter opening/closing means must follow theshutter opening/closing sequence of FIG. 46 b at an unlimited rate, butexhibits relative light transmittance characteristics due to the limitedlight response characteristics thereof, as shown in FIG. 46 c. Due tothe limited light response of the shutter opening/closing means, in aboundary region where P and M images are alternated with each other, theuser views a masking image that must have been blocked, or a darkprivate image that must be viewed. Since it is seriously problematic forthe user to view a masking image, the viewing of the masking image mustbe avoided.

In the present invention, as shown in FIG. 47, a shutter response timefor shutter opening/closing sequence is calculated, and the user is madeto view a P image or a connecting image Mb, such as a blank image, forthe shutter response time. As shown in FIG. 47 a, in a boundary regionwhere P and M images are alternated, a private image P or connectingimage Mb is provided and displayed for the shutter response time. Withthis operation, a user is prevented from viewing a masking image thatmust have been blocked.

Additionally, in order to solve the problem of the difference in thedensity of light intensity due to a limited shutter response time, thepresent invention applies a combination of a P image grouping method anda boundary region processing method. For an embodiment, FIG. 48illustrates shutter opening/closing sequence light responses and aboundary region processing method according to region division imagearrangement with respect to monitor frames 1 and 2 that are privateimage grouping units. In the above case, all boundary regions are filledwith P images. The user views a boundary region in the upper portion ofa monitor while a shutter is being opened in the monitor frame 1, orwhile the shutter is being closed in the monitor frame 2, so thatapproximately one private image is viewed through a maximally openedshutter on the average. With this method, the problem of the differencein the density of light intensity is overcome. The boundary region inthe lower portion of the monitor is compensated for in the same manner.

For another embodiment, FIG. 49 illustrates shutter opening/closingsequence light responses and a boundary region processing methodaccording to region division image arrangement with respect to monitorframes 1 and 2 that are private image grouping units. In the above case,all boundary regions are filled with connecting images Mb. As indicatedby arrows in the drawing, the start and end positions of a P image in aframe 1 are made to accurately coincide with the end and start positionsof a P image in a frame 2, respectively. With this method, the problemof the difference in the density of light intensity is overcome. Thismethod is superior in ‘user visual perception performance,’ but isinferior in security in the region where the connecting image Mb isdisplayed. Accordingly, in another monitor frame, a masking image shouldbe sufficiently provided for this region. When a method using theconnecting image Mb is employed, a region division image arrangementsequence, in which a boundary region is continuously varied, isgenerated and displayed in an embodiment.

The above described embodiments are disclosed only to allow thoseskilled in the art to easily understand and work the present invention,but are not disclosed to limit the scope of the present invention.Accordingly, those skilled in the art should note that variousmodifications and alterations are possible without departing from thescope of the invention. The scope of the present invention isprincipally defined by the claims that will be described later.

INDUSTRIAL APPLICABILITY

In accordance with the present invention described above, an effectivemasking image can be generated with human visual perceptioncharacteristics taken into consideration. Furthermore, an originalimage-derived image, such as a dynamic inverse image, and a disturbingimage can be generated in real time.

Furthermore, in accordance with the private image and masking imagemixing method of the present invention, user visual perceptionperformance and security performance can be simultaneously improved in aprivate image output apparatus.

Furthermore, in accordance with the present invention, user visualperception performance is not only fulfilled, but also anti-peepersecurity performance can be further improved.

1. An apparatus for outputting a private image using a public display,comprising: means for generating at least one private image; means forgenerating at least one masking image that masks the private image;means for generating an image sequence of the private image and themasking image; and means for outputting the private image and themasking image to the display according to the image sequence; whereinthe masking image generating means generates a dynamic inverse image ofthe private image as the masking image according to a refresh rate ofthe display and the image sequence.
 2. The apparatus as set forth inclaim 1, further comprising shutter opening/closing means, the shutteropening/closing means including a shutter, and a shutter control unitadapted to open and close the shutter according to a shutteropening/closing sequence corresponding to the image sequence.
 3. Theapparatus as set forth in claim 2, wherein the shutter opening/closingmeans further comprises an ambient light blocking filter.
 4. Theapparatus as set forth in claim 1, wherein the masking image generatingmeans calculates a mean image value G_(ad), and then calculates adynamic inverse image value G_(ai) by applying the calculated mean imagevalue G_(ad) to the following equation.G _(ai) ^(γm)=2G _(ad) ^(γm) −G _(ap) ^(γm) or G _(ai) ^(γm) =G _(a max)^(γm) +G _(a min) ^(γm) −G _(ap) ^(γm) where G_(ai) is the dynamicinverse image value, G_(ad) is the mean image value, G_(a max) is amaximum image value, G_(a min) is a minimum image value, and γ_(m) is agamma value of the private image outputting apparatus
 5. The apparatusas set forth in claim 1, wherein the masking image generating means a)calculates an approximation of the mean image value, b) selects a meanimage value at which clarity of a pattern is lowest while performingdisplay to alternate original images, in which the pattern has theapproximation of the mean image value and a remaining portion has aminimum image value G_(a min) or a maximum value G_(a max), with inverseimages of the original images, in which the pattern has theapproximation of the mean image value and a remaining portion has themaximum value G_(a max) or the minimum image value G_(a min), c)approximately calculates a dynamic inverse value from the followequation,G _(ai) ^(γm)=2G _(ad) ^(γm) −G _(ap) ^(γm) where G_(ai) is the dynamicinverse image value, G_(ad) is the mean image value, G_(ap) is a privateimage value and γ_(m) is a gamma value of the private image outputtingapparatus d) selects the dynamic inverse value at which clarity of apattern is lowest while performing display to alternate original images,in which the pattern has the mean image value and a remaining portionhas G_(ap) or G_(ai), with inverse images of the original images, inwhich the pattern has the mean image value and a remaining portion hasG_(ai) or G_(ap).
 6. The apparatus as set forth in claim 1, wherein themasking image generating means generates the masking image by generatinga disturbing image based on human visual perception characteristics andmixing the disturbing image with the dynamic inverse image.
 7. Theapparatus as set forth in claim 1, wherein the masking image generatingmeans generates a colored dynamic inverse image.
 8. An apparatus foroutputting a private image using a public display, comprising: means forgenerating at least one private image; means for generating at least onemasking image that masks the private image; means for generating animage sequence of the private image and the masking image; and means foroutputting the private image and the masking image to the displayaccording to the image sequence; wherein the masking image generatingmeans generates a disturbing image based on human visual perceptioncharacteristics as the masking image.
 9. The apparatus as set forth inclaim 8, wherein the masking image generating means further generates animage derived from the private image according to a refresh rate of thedisplay and the image sequence, and generates the masking image byadding the derived image and the disturbing image.
 10. The apparatus asset forth in claim 8, wherein the masking image generating meanscomprises a disturbing image generating unit configured to generate anappropriate disturbing image according to a disturbing image generationpolicy based on necessary visual perception characteristics.
 11. Theapparatus as set forth in claim 10, wherein: the masking imagegenerating means further comprises a storage unit storing the disturbingimage with an index for the disturbing image set up based oncharacteristics of the disturbing image; and the disturbing imagegenerating unit searches the storage unit for an appropriate disturbingimage and generates the disturbing image according to the disturbingimage generation policy.
 12. The apparatus as set forth in claim 10,wherein the disturbing image generating unit examines characteristics ofthe private image and sets up the disturbing image generation policy.13. The apparatus as set forth in claim 10, wherein the disturbing imagegenerating unit generates the disturbing image including importantspatial and temporal frequencies of the private image.
 14. The apparatusas set forth in claim 10, wherein the disturbing image generating unitgenerates the disturbing image fulfilling predetermined frequencycharacteristic conditions to include frequency components, to whichhuman visual perception is sensitive, at a specific level or more. 15.The apparatus as set forth in claim 10, wherein the disturbing imagegenerating unit generates the disturbing image in which anyone ofluminance and color contrasts of each of image components constitutingthe disturbing image is higher than a predetermined value.
 16. Theapparatus as set forth in claim 15, wherein the disturbing imagegenerating unit generates the disturbing image in which one of luminanceand color contrasts of each of image components constituting thedisturbing image is higher than that of the private image.
 17. Theapparatus as set forth in claim 10, wherein the disturbing imagegenerating unit generates the disturbing image through combination oftwo or more images having sensitive frequency, contrast and cognitivecontent characteristics.
 18. The apparatus as set forth in claim 17,wherein the combination is performed in such a way as to arrange imageshaving different characteristics in frames regions of the display,respectively.
 19. The apparatus as set forth in claim 17, wherein thecombination is performed in such a way as to overlap images havingdifferent characteristics in a region of the display.
 20. The apparatusas set forth in claim 10, wherein the disturbing image generating unitgenerates the disturbing image so that visual perception is moresensitive to a specific region of the display than to other regionsthereof.
 21. The apparatus as set forth in claim 20, wherein thedisturbing image generating unit generates the disturbing image so thatthe visual perception is more sensitive to a center region of thedisplay than to upper and lower regions of the display.
 22. Theapparatus as set forth in claim 10, wherein the disturbing imagegenerating unit generates the disturbing image that includes an imagecomponent having a characteristic similar to that of the private image.23. An apparatus for outputting a private image using a public display,comprising: means for receiving at least one private image from anexternal server or generating a private image by itself; means forreceiving at least one masking image from an external server; means forgenerating an image sequence of the private image and the masking image;and means for outputting the private image and the masking image to thedisplay according to the image sequence.
 24. An apparatus for outputtinga private image using a public display, comprising: means for receivingat least one private image from an outside of the apparatus; means forgenerating at least one masking image that masks the private image;means for generating an image sequence of the private image and themasking image; and means for outputting the private image and themasking image to the display according to the image sequence.
 25. Theapparatus as set forth in claim 24, wherein the image outputting meanscompresses a range of image values of the private image relative to thatof the masking image.
 26. The apparatus as set forth in claim 24,wherein the masking image generating means generates a dynamic inverseimage of the private image as the masking image according to a refreshrate of the display and the image sequence.
 27. The apparatus as setforth in claim 26, wherein the masking image generating means iscomposed of an inverter configured to invert the private image.
 28. Theapparatus as set forth in claim 24, wherein the masking image generatingmeans generates a disturbing image based on human visual perceptioncharacteristics as the masking image.
 29. An apparatus for outputting aprivate image using a public display, comprising: means for receiving atleast one private image from an outside of the apparatus; means forgenerating at least one masking image that masks the private image;means for receiving an image sequence of the private image and themasking image; and means for outputting the private image and themasking image to the display according to the image sequence.
 30. Theapparatus as set forth in claim 29, wherein the image outputting meanscompresses image values of the private image relative to those of themasking image.
 31. An apparatus for outputting a private image using apublic display, comprising: means for generating at least one privateimage; means for generating at least one masking image that masks theprivate image; means for generating an image sequence of the privateimage and the masking image; and means for outputting the private imageand the masking image to the display according to the image sequence;wherein the masking image generating means comprises a color tablestorage unit having a color table for generation of the masking imageand an image conversion unit generating the masking image with referenceto the color table for generation of the masking image.
 32. Theapparatus as set forth in claim 31, wherein the color table storage unitincludes a color table for generation of the masking image that convertsthe private image into the masking image, and the image conversion unitgenerates the masking image through conversion of the private image withreference to the color table for generation of the masking image. 33.The apparatus as set forth in claim 31, wherein the masking imagegenerating means comprises; a disturbing image generation unitgenerating the disturbing image; and an image conversion unit generatingthe masking image using an image obtained by converting the disturbingimage with reference to the color table for generation of the maskingimage.
 34. The apparatus as set forth in claim 31, further comprising: acolor table storage unit having a color table for generation of theprivate image; and an image conversion unit generates the private imagewith reference to the color table for generation of the private image.35. The apparatus as set forth in claim 34, wherein the image outputtingmeans compresses a range of image values of the private image relativeto that of the masking image in such a way as to set ranges of imagevalues of the color table for generation of the masking image and thecolor table for generation of the private image.
 36. A method ofoutputting a private image using a public display and a shutter,comprising: selecting a rule for mixing private at least one image andat least masking image to fulfill security performance and user visualperception performance; generating an image sequence of the privateimage and the masking image according the mixing rule; generating ashutter opening/closing signal corresponding to the image sequence tofulfill the user visual perception; outputting the private image and themasking image to the display according to the image sequence; andopening/closing the shutter according to the shutter opening/closingsignal.
 37. The method as set forth in claim 36, further comprisingauthenticating a user and receiving a level of the security performanceof the display.
 38. The method as set forth in claim 36, furthercomprising: determining whether the shutter opening/closing signalfulfills the user visual perception performance; re-selecting the mixingrule if the shutter opening/closing signal does not fulfill the uservisual perception performance; and re-generating the image sequence andthe shutter opening/closing sequence according to the re-selected mixingrule, outputting the re-generated image sequence, and opening/closingthe shutter according to the shutter opening/closing sequence.
 39. Themethod as set forth in claim 36, wherein the mixing rule is selected inview of a refresh rate and pixel response time of the display, a lightresponse time of the shutter and characteristics of the private imageand the masking image.
 40. The method as set forth in claim 36, whereinthe generating of the shutter opening/closing signal is performed togenerate the shutter opening/closing signal including at least onesignal opening/closing the shutter at an intermediate state.
 41. Amethod of outputting a private image using a public display and ashutter, comprising: generating an image sequence of at least oneprivate image and at least one masking image using a method of insertingphase change aperiodicity by limiting a maximum allowable repetitivenumber of unit repetitive periods each composed of at least one privateimage and at least one masking image; generating a signal to open orclose the shutter according to the image sequence, so as to fulfill uservisual perception performance; outputting the private image and themasking image to the display according to the image sequence; andopening/closing the shutter according to the shutter opening/closingsignal.
 42. The method as set forth in claim 41, wherein the generatingof the image sequence is performed so that repetitive periods, whosenumber is equal to or less than the maximum allowable repetitive number,are randomly arranged.
 43. The method as set forth in claim 41, whereinthe generating of the image sequence is performed so that the maximumallowable repetitive number varies with time.
 44. The method as setforth in claim 41, wherein the generating of the image sequence isperformed in such a way that the unit repetitive period varies withtime.
 45. The method as set forth in claim 41, wherein the generating ofthe image sequence is performed in such a way that the phase changeaperiodicity is generated 15 or less times per second or 25 or moretimes per second.
 46. A method of outputting a private image using apublic display and a shutter, comprising: generating an image sequenceof at least one private image and at least one masking image using amethod of limiting a maximum allowable repetitive number of imageshaving a same characteristic; generating a shutter opening/closingsignal corresponding to the image sequence to fulfill user visualperception performance; outputting the private image and the maskingimage to the display according to the image sequence; andopening/closing the shutter according to the shutter opening/closingsignal.
 47. The method as set forth in claim 46, wherein the generatingof the image sequence is performed in such a way that it is determinedin detail based on a characteristic of the masking image whether theimage has the same characteristic.
 48. A method of outputting a privateimage using a public display and a shutter, comprising: generating animage sequence of at least one private image and at least one maskingimage using a method of inserting phase change aperiodicity by limitinga maximum allowable repetitive number of unit repetitive periods eachcomposed of at least one private image and at least masking image whilelimiting a maximum allowable consecutive number of images having a samecharacteristic; generating a shutter opening/closing signalcorresponding to the image sequence to fulfill user visual perceptionperformance; outputting the private image and the masking image to thedisplay according to the image sequence; and opening/closing the shutteraccording to the shutter opening/closing signal.
 49. A method ofoutputting a private image using a public display and a shutter,comprising: generating an image sequence of at least one private imageand at least one masking image using a method of inserting phase changeaperiodicity by limiting a maximum allowable repetitive number of unitrepetitive periods each composed of at least one private image and atleast masking image; generating a shutter opening/closing signalcorresponding to the image sequence to fulfill user visual perceptionperformance; outputting the private image and the masking image to thedisplay according to the image sequence; and opening/closing the shutteraccording to the shutter opening/closing signal; wherein the generationof the shutter opening/closing signal is performed in such a way thatthe shutter is opened/closed at an intermediate state in a vicinity of atime when the phase change aperiodicity is inserted.
 50. The method asset forth in claim 49, wherein the generating of the image sequence isperformed in such a way that a connecting image frame is generated asthe masking image corresponding to the intermediate state of theopening/closing of the shutter.
 51. The method as set forth in claim 49,wherein the shutter opening/closing signal of the intermediate state ofthe opening/closing of the shutter continuously varies an extent of theopening/closing of the shutter with an lapse of time for a single imageframe period.
 52. A method of outputting a private image using a publicdisplay and a monitor, comprising: selecting a rule of arrangement of atleast one private image and at least one masking image to mask aspecific region of the monitor; generating an image sequence of theprivate image and the masking image according the arrangement rule;generating a shutter opening/closing signal corresponding to the imagesequence to mask a specific region of the monitor; outputting theprivate image and the masking image to the monitor according to theimage sequence; and opening/closing the shutter according to the shutteropening/closing signal.
 53. The method as set forth in claim 52, whereinthe masked region is scrolled.
 54. A method of outputting a privateimage using a public display and a monitor, comprising: selecting a rulefor arranging private images and masking images to be alternated witheach other at least two times for a single monitor frame; generating animage sequence of private images and masking images according thearrangement rule; generating a shutter opening/closing signal tocorrespond to the image sequence; outputting the private image and themasking image to the monitor according to the image sequence; andopening/closing the shutter according to the shutter opening/closingsignal.
 55. The method as set forth in claim 54, wherein the selectionof the arrangement rule is performed in such a way that M private imagesare grouped for N frames of the monitor wherein M is less than N. 56.The method as set forth in claim 54, wherein the alternation of theprivate images and the masking images is synchronized with vertical orhorizontal sync of the monitor.
 57. The method as set forth in claim 54,wherein the alternation of the private images and the masking images issynchronized with not the monitor but only a data sync.
 58. The methodas set forth in claim 54, wherein the shutter opening/closing sequenceis synchronized with the alternation of the image sequence.
 59. Themethod as set forth in claim 54, wherein the private images and themasking images have a cycle time that varies with time.
 60. The methodas set forth in claim 54, wherein a grouping unit composed of the Nmonitor frames and the M private images varies with time.
 61. The methodas set forth in claim 54, wherein a grouping unit composed of the Nmonitor frames and the M private images is fixed, and numbers of theprivate images and the masking images arranged for a single monitorframe are variable.
 62. The method as set forth in claim 61, whereinregion division boundaries formed by arranging the private images andthe masking images for the single monitor frame coincide with each otherwith respect to the grouping unit.
 63. A method of outputting a privateimage using a public display and a monitor, comprising: selecting a rulefor arranging private images and masking images to be alternated witheach other at least two times for a single monitor frame; generating animage sequence of private images and masking images according thearrangement rule; generating a shutter opening/closing signal tocorrespond to the image sequence; outputting the private image and themasking image to the monitor according to the image sequence; andopening/closing the shutter according to the shutter opening/closingsignal; wherein masking images having different characteristics areoutput to regions of the monitor.
 64. The method as set forth in claim63, wherein the selection of the arrangement rule is performed in such away that M private images are grouped for N frames of the monitorwherein M is less than N.
 65. The method as set forth in claim 63,wherein the selection of the arrangement rule is performed to arrangethe private images and the masking images so that a masking image havinga strong masking effect is output to a center region of the monitor andmasking images having a weak masking effect are output to upper andlower regions of the monitor.
 66. The method as set forth in claim 65,wherein the selection of the arrangement rule is performed so that amixed image of an original image-derived image and a disturbing image isoutput to a center region of the monitor and original image-derivedimages are output to upper and lower regions of the monitor.
 67. Themethod as set forth in claim 63, wherein the outputting of the images isperformed so that a masking image being relatively rapidly updated isoutput to a center region of the monitor and masking images beingrelatively slowly updated are output to upper and lower regions of themonitor.
 68. A method of outputting a private image using a publicdisplay and a monitor, comprising: selecting a rule for arrangingprivate images and masking images to be alternated with each other atleast two times for a single monitor frame; generating an image sequenceof private images and masking images according the arrangement rule;generating a shutter opening/closing signal to correspond to the imagesequence; outputting the private image and the masking image to themonitor according to the image sequence; and opening/closing the shutteraccording to the shutter opening/closing signal; wherein a private imageor connecting image is output for a response time of the shutter. 69.The method as set forth in claim 68, wherein, between monitor frames inwhich the private images are grouped, start and end locations of theshutter response time coincide with each other, and during the shutterresponse times private images are output.
 70. The method as set forth inclaim 68, wherein, between monitor frames in which the connecting imagesare grouped, an end position of a shutter response time of an oddmonitor frame coincides with a start position of the shutter responsetime of an even monitor frame and an start position of the shutterresponse time of the odd monitor frame coincide with an end position ofthe shutter response time of the even monitor frame, and during theshutter response times, private images are output.